I currently have a 9" booster that came with my disc brake conversion. It's not stopping they way I'd hoped. Pedal is hard and doesn't stop very well. I've checked everything, all is good no air in system, and M/C is good with correct push rod and clevis in correct postion on pedal. vacuum is 11" hg in gear not the best but should be enough.

Would going to an 11" booster be a big enough improvement over the 9" to warrant the expense to switch?

Jim

I currently have a 9" booster that came with my disc brake conversion. It's not stopping they way I'd hoped. Pedal is hard and doesn't stop very well. I've checked everything, all is good no air in system, and M/C is good with correct push rod and clevis in correct postion on pedal. vacuum is 11" hg in gear not the best but should be enough.

Would going to an 11" booster be a big enough improvement over the 9" to warrant the expense to switch?

Jim

My friend has a 65 Chevelle with the same issue. He has a 9 inch booster added during a disk brake swap to new post 67 discs. He has almost no stopping power after one good hard stop from speed and he claims that even the first hard stop is not up to his expectations. He is VERY hard to please so I suppose that might be a factor. . Don't know about 67 cars but the 65 is VERY close to having insufficient clearance at the inner fender when running the 11 inch. I saw one (a 65 ) at a show and it was OH SO close to the inner fender.

BTW my buddy has a 350 with the relatively mild L-79 327 cam. He has been cussin that 9 inch booster for YEARS and swears he is going to fix it this winter.

Have you consided a dual diaphram (sp?) 9 inch unit as opposed to the 11 incher?

I would try one if I thought it would work. I need a booster rental store.

Looks like any interference would come from hitting the valve cover but I think it will be clear.

Have you though about a vacuum canister. I had the same problem when I switched to the 454. I also have a 9" booster. The comp cams canister made a big difference. Not like it was with the stock small block but better for less money.

Shows the difference between the 9" booster and 11 in. booster. F11 on the keyboard will help some to enlarge photo. Shows single and duel diaphram bosters.

A different booster isn't going to make the car stop better, it just lessens the amount of muscle needed to push the pedal down--I have manual brakes on one of my cars and it stops just fine from 130+. If you want it to lock the brakes with one toe on the pedal, then you can play with sizing the booster to suit. If the car stops worse after the first hard stop, it's probably brake fade and a different booster isn't going to change that.
If all else fails, maybe a gym membership will help....work on those leg muscles!:D

A different booster isn't going to make the car stop better, it just lessens the amount of muscle needed to push the pedal down

If you want it to lock the brakes with one toe on the pedal, then you can play with sizing the booster to suit.

If the car stops worse after the first hard stop, it's probably brake fade and a different booster isn't going to change that.
D

I gotta ask a few questions based on the three points above.

#1 If the booster lowers the amount of force needed from you foot is the following illogical? Assume the same car with a booster and without. No other changes. Is it not logical to assume that the SAME amount of input pressure applied without a booter in the car and with a booster in the car will produce more pressure at the brakes with the booster?

Adding the booster does not change the inherent stopping power of the brakes...agreed. The brakes are what they are...disc size, swept area number of pistons etc etc etc. No change in booster is gonna change the ability of the disc / caliper pair to stop the car. I think the issue here is the maximum utilization of the inherant capacity of the disc and caliper to do whatever amount of work they are capable of doing

#2 I don't read anything about locking the brakes with one toe on the pedal. I think the isuse with this 67 is as it is with my friends car....just not enough stopping power. This is not about locking the brakes.

#3 My 72 has the same brakes that my friends 65 has. For the record his system is fully rebuilt with new calipers, new "pucks" etc etc etc. We have done tests...stopping both cars from the same speed, starting at the same position on the road and measuring the distance to where the car stops moving.

My car (11 inch booster) consistantly stops in a shorter distance. Time after time (for the record we have never tried 130 MPH test stops) my car stops in a shorter distance. On the first test his car takes a longer distance to stop than than mine does. On subsequent stops my car experiences some increase in total distance required (fade?). His car experience much more increase in distance after the first stop than mine does. Time between tests for both cars is the same to adjust for heat disipation.

Our theory as to why his takes more distance on the subsequent stops is as follows. His front brakes are not working as well as they should. That is why he takes a longer distance on the first stop. His rear brakes are working harder than mine..because his fronts are not doing what they should do...providing most of the stopping power. He is getting most of his stopping power from the rears. On each subsequent stop his rear brakes are gettting hotter and fading more and more...hence longer stopping distances on each test. On the other hand my car has properly function front brakes. My rear brakes are not being asked to do as much of the stopping as his are. Thereofore my car does not experince the increases in distance that his does.

If the problem under consideration here is related to brake fade how do we explain two cars with same set of brakes (front and rear) having different results? If you are right my car should track more or less the same as his as far as increased distance on each subsequent stop. Said another way: If his brakes are fading then why are mine not fading at more or less the same rate as his? He and I think the answer is that his fronts are not performing up to par as I outlined above. Remember he has the same brakes (disc size and caliper) the only difference is the size of the booster.

This discussion is a variation on another argument that I read in the Vette Forums. People wanna convert the 63 and 64 non disc cars to disc. There are people who maintain that there is no appreciable gain to be had going to discs. They defend to the death the following hypothesis "Well maintained drums are as good as discs". I can't buy that arguement. Peoperly function discs are better than drums any day. Beter in the wet, better in repeated stops etc. etc etc.

My logic continues that way in terms of this problem. Non power assisted disc equipped cars require more foot pressure than booster assisted disc equipped cars. That leads me to the conclusion that adding a booster helps apply more pressure at the brake from a given amount of pressure at the pedal.

I would appreciate your obesrvations and comments on the three points outlined above. I have heard sooooo much from my friend about lousy brakes and any expert information on this point would be great.

Jon, I'm glad to hear that. I've been getting mixed answers on these canisters actually helping. For $35 it's worth a try.

Roy, thank you for the chart.

I really think my issue lies with the combination of less than desirable vacuum and a 9" booster.

mike, manual brakes are a strong consideration if nothing else works.

Jon, I'm glad to hear that. I've been getting mixed answers on these canisters actually helping. For $35 it's worth a try.

Roy, thank you for the chart.

I really think my issue lies with the combination of less than desirable vacuum and a 9" booster.

mike, manual brakes are a strong consideration if nothing else works.

I have to say that my buddy has tried all the "standard" fixes for this. Don't wanna rain on your parade but the Vac canister did nothing for the issue with my friends car. IMHO save the $35 and use it for an 11 inch booster or for a dual diaphram booster.

My simple deductive logic mind asks the following "What is different between the brake systems on my friends car and mine?" The difference is the booster! In the lexicon of the Bill Clinton campaign I have to conclude "It is the BOOSTER... stupid"

PLEASE NOTE that is a play on words NOT a comment on any post in this thread. Cars of roughly equal weight, running the same front and rear brakes subjected to the same test perform differently...what is the logical path to the solution? Look at what is different! My car is black my friends is green...Nope that aint it. I have a 5 Speed he has a 4 Speed...nope that aint it.

"It is the booster" stupid comes right at me on this problem. Again NOT a slam at anyone just a play on words.

oman,
Consider this: You have not mentioned a word about the engine specs (cam profile, timing, carburetor) all of which affect the vacuum present at idle. This is the MAJOR factor in determining how your power brakes will work differently than your friends.

oman,
Consider this: You have not mentioned a word about the engine specs (cam profile, timing, carburetor) all of which affect the vacuum present at idle. This is the MAJOR factor in determining how your power brakes will work differently than your friends.

Yes I agree. Also I don't think we know what cam the guy who asked the original question is running. Did I miss that? I think my premise about the booster still holds water however. If my engine and my friends are as described below, not running killer cams, and his brakes don't work then the conclusion that a wild cam (which the original poster might indeed be running) will only make a bad situation like what my friend is seeing worse seems valid. Remember if this occurs with a mild cam laying the problem off on the cam is not likely a valid conclusion.

I was just thinking about that cam question as I was sitting here. In point of fact my friends engine is more likely to be vac booster friendly than mine. He runs a mild 350 engine. He is running a L-79 327 350 HP Chevy cam. Certainly that is not a wild cam by any stretch of the immagination. I have a more or less mild 454 ...LS6. I know that some folks wanna think that the LS6 is the biggest meanest killer since Attila the Hun but in point of fact the cam in the stock LS6 is more or less mild compared to some of the aftermarket stuff out there. A note here..my LS6 is the 1971 Corvette version with 9.1 to 1 CR and my buddies engine is also a low 9.nn to 1 Compression ratio engine.

Neither my car or my friends car run a "killer cam" which cannot produce enough vac. I don't have specifications on both cams handy. My engine runs solid lifters and his has hydraulic but in and of itself that proves nothing. The lobe center dimension and other cam factors are the issue not the lifter type.

If anything both his engine and mine are on the friendlier side of the cam profile / vac curve. If I had to give an opinion I would say his engine would be the one of the two that I would least suspect to have insufficient vac to operate the booster.

So where we get into the arguing match here is simply this:

If the driver is able to generate sufficient brake pressure at the wheels to lock the wheels, then the driver can achieve the maximum braking performance the combination is capable of giving.

This does not change if the driver inputs 200 lbs of force at the brake pedal to generate that wheel locking pressure or 600 lbs of force. Thus the claim that booster size and vacuum reservoir matter not.

If the driver is unable to generate the pedal force required to generate the brake pressure at the wheels required for lockup, then boost assist will reduce stopping distance by effectively increasing brake pressure at the wheels.

Oman I would ask if you and your friend have validated your testing results by actually locking up the brakes? If not it is likely that you are getting closer to the lockup threshold in your car than your friend is in his.

Both disk and drum can generate the required stopping force just fine. The advantages of disks are superior heat dissipation, shorter response time, and faster modulation capability. Drum brakes have an advantage of requiring less pressure for a given amount of brake force.

So where we get into the arguing match here is simply this:

If the driver is able to generate sufficient brake pressure at the wheels to lock the wheels, then the driver can achieve the maximum braking performance the combination is capable of giving.

This does not change if the driver inputs 200 lbs of force at the brake pedal to generate that wheel locking pressure or 600 lbs of force. Thus the claim that booster size and vacuum reservoir matter not.

If the driver is unable to generate the pedal force required to generate the brake pressure at the wheels required for lockup, then boost assist will reduce stopping distance by effectively increasing brake pressure at the wheels.

Oman I would ask if you and your friend have validated your testing results by actually locking up the brakes? If not it is likely that you are getting closer to the lockup threshold in your car than your friend is in his.

Both disk and drum can generate the required stopping force just fine. The advantages of disks are superior heat dissipation, shorter response time, and faster modulation capability. Drum brakes have an advantage of requiring less pressure for a given amount of brake force.

First: I am not sure we are arguing.

"Oman I would ask if you and your friend have validated your testing results by actually locking up the brakes? If not it is likely that you are getting closer to the lockup threshold in your car than your friend is in his."

I suppose I can lock the wheels on my car. Honestly though I gotta say that I never tried. I suspect that my friends 65 won't lock the brakes. I have been in the car in a few tests and honestly I don't think the car can lock the front wheels. Any testing we did was not aimed at taking the wheels to lock.

It indeed is likely that I am closer to the lockup threshold than my friend is. If that is true I am closer because my booster is capable of taking me closer to lockup than his is. As I said I think this is why his brakes fade more than mine do. I get closer to the lockup point therefore I am using the front brakes more than he is. He is asking his rears to do more work than I am and they are fading faster as a result.

"If the driver is unable to generate the pedal force required to generate the brake pressure at the wheels required for lockup, then boost assist will reduce stopping distance by effectively increasing brake pressure at the wheels."

Agreed. Boost assistance will effectively increase braking pressure at the wheels. This assumes that the boost assistance mechanism is indeed working. That I think is the crux of this discussion..."Does the 9 inch booster provide sufficient boost to operate later model discs?".

Remember this, 65's never came with discs. He has a booster designed for a 65 Chevelle that had drum brakes. He has that 9 inch booster because the 11 incher is a tough fit in the 65 engine compartment. I suspect, but I cannot say for sure, that the 1965 9 inch drum booster cannot operate the later 72 discs to their max capability.

"Both disk and drum can generate the required stopping force just fine. The advantages of disks are superior heat dissipation, shorter response time, and faster modulation capability." Agreed

"Drum brakes have an advantage of requiring less pressure for a given amount of brake force" Ok..

If this is so please take the time to follow this line of thought thru. Say that drums require less pressure (I assume you mean pedal pressure) for a given amount of force (I assume that you mean stopping force). Do you then agree that discs need more pedal pressure for a given amount of stopping force than drums? All I did was turn your sentence around when writing my sentence. I am simply speaking about the disc in terms of the drum. No slight of hand there.

My thought is that a 9 inch booster designed to produce sufficient pressure for a set of drums may be incapable of generating sufficient force to operate a set of discs at their optimum capability. If we assume for the purpose of this discussion that the preceeding sentence is true then we are back to someones comment that "You need to go to the gym to strengthen your leg". Why? Because the booster is not capable of working the discs to produce the stopping force they have the potential to produce. Since the booster can't do it "you need a stronger leg" has to be the conclusion we reach.

Why would one want to stess and strain and grunt and groan to overcome the limitations of a 9 inch booster that cannot correctly operate discs? What we need to verify or disprove all this is the following. For a given amount of force at the pedal what force can the 9 inch and 11 inch boosters produce? Then we need to know how much force is needed at the drum and at the disc to produce the maximum stopping force that each system is capable of.

With those numbers we know how much output the 9 inch booster can generate and we know how much input is needed by the discs to operate at their maximum. If the 9 inch cannot generate sufficient boost to operate the discs and make the discs produce at least as much force as the drums can produce (let alone as much as the discs can produce) we have answered our question. Trouble is we don't have those numbers.

I think the 9 inch booster cannot operate the discs at their capability because it cannot generate as much boost as the 11 inch booster can. I think it is likely that the 9 inch booster cannot even get the discs to produce as much force as it can get the drums to produce

A little data for the "step harder on the brake after you go to the gym" proponents.

I called my friend last night to talk a little about this thread. He and I had not talked in about 3 months and this issue was a good excuse to make a call. It is almost winter in the North East and he is getting ready to fix the brake problem over the down time that bad roads cause in the "snow belt" His continued research has added some numbers to the empty "equation" we had earlier.

He is determined to get this right this time and not rely on the brake conversion vendor <Who shall remain nameless> suggestions that "You don't need an 11 inch booster or a dual dia 9 inch booster". BTW he wanted a dual dia 9 inch booster from the get go and they discouraged him from buying one. I suspect they discouraged him because the 11 incher did not fit well and / or because the dual 9 inch was more expensive than the single 9 dia 9 inch. The added cost of the dual dia 9 inch might have cost the vendor the conversion kit sale.

According to his findings:

The 11 inch brake booster is capable of 1100 +/- lbs of force at the caliper / drum.

The 9 inch brake booster is capable of approximately 800 +/- lbs of force at the caliper / drum. Do the math exercise fans .....it would appear that there is a shortage of about 300 lbs of pressure at the discs with the 9 inch booster.

The discs he and I have are designed to operate with the 11 inch booster at approximately 1100 LBS. Difference between the two cars is that I have the 11 inch booster, he doesn't. BTW he has also found that the DUAL dia. 9 inch booster is good for over 1100 lbs.

Gotta sign off now...headed to the gym.

How do you explain a race cars ability to stop without a booster? Ever watch a Pro Stock driver stage? A quick pump on the pedal to get 1000psi to hold the car still. I have no trouble locking the brakes on my manual discs.

The 11 inch brake booster is capable of 1100 +/- lbs of force at the caliper / drum.

The 9 inch brake booster is capable of approximately 800 +/- lbs of force at the caliper / drum. Do the math exercise fans

How much input is being used to generate those figures? What happens if you push harder? Does the pedal suddenly hit a point where it won't apply more force?

How about the proportioning valves on your car vs. your buddy? Are they the same? Do the cars weigh exactly the same? Do you have the exact same wheel and tire combo? Heavy steel wheels take a lot more effort to stop than some nice light alloy pieces--Springs and shocks? Does one stay flatter while braking? Does one do a nosedive into the pavement?
Check road tests in different magazines--stopping distance vary widely..road surface, temperature.....far too many variables.

How do you explain a race cars ability to stop without a booster? Ever watch a Pro Stock driver stage? A quick pump on the pedal to get 1000psi to hold the car still. I have no trouble locking the brakes on my manual discs.



How much input is being used to generate those figures? What happens if you push harder? Does the pedal suddenly hit a point where it won't apply more force?

How about the proportioning valves on your car vs. your buddy? Are they the same? Do the cars weigh exactly the same? Do you have the exact same wheel and tire combo? Heavy steel wheels take a lot more effort to stop than some nice light alloy pieces--Springs and shocks? Does one stay flatter while braking? Does one do a nosedive into the pavement?
Check road tests in different magazines--stopping distance vary widely..road surface, temperature.....far too many variables.

Come on man!!!!!

I dunno why you want to believe this theory that "just pressing harder" is the solution. You spit out all these variables asking questions that only a vehicle dynamics engineer can answer after emperical work with two INSTRUMENTD cars and after a long set of calculations. YES YES YES all the of this stuff you mention could explain the WIDE desparity in the way the cars perform in the same road condition. Trouble is we can't get there from here.

The fact is that one car ALWAYS stops in a shorter distance. The fact is that on subsequent stops the car with the longer original stopping distance shows greater growth is stopping distance than the other car does. You are getting into areas that require a true engineering analysis / test of ALL the coefficients and variable parameters in the two cars.

I have the 72 Porportioning valve. I assume that he has the porportioning vale recommended by the namelss conversion kit vendor. Both cars run Chevy rally wheels, I have 15 x8 on the rear and 15 x7 in the front. He has 15 x7 all around. I don't know the tread patern, rubber compounds and the tire manufacturers...I will work to get those for you. Springs...the springs are XYZ steel manufactured by ABC Spring Company in Detroit. My cars has 6.125 coils and his has 7.009 coils in each front spring. Each is wound in ..33681inch dia steel. The road surface for the tests was 3000 psi concrete in some cases and asphalt in others. Ambient temperature was 69 F during the concrete test and 71.6 F during the concrete test. Humidity was 75% in some tests and 76% in others. In all cases winds were light and variable blowing predominately from the front of the cars to the rear.

I don't explain race cars "quick pump.........." Who knows what custom designed, purpose built stuff is in those cars? Not me. Willwood this and XYZ that could be installed in those cars. The "how do you explain.......' has no bearing on this discussion. All it does is cloud the issue just like the litany of listed specifications above serves to obviscate the situation.

Yes ALL those factors do effect braking... no doubt. That said we have two cars running stock GM braking componets, weight essentially the same, each running street compound tires, each stopping on the same surface. One ALWAYS takes further to stop. One ALWAYS experiences MORE growth in stopping distance than the other car does during subsequent stops.

All the engineering factors you site in your litany of questions about components sounds like an really savy engineering analysis is planned for this problem. At the same time you ask "What happens when the booster....does it suddenly...." The booster like any other mechanical component does have specifications as to what it can do. The boosters are rated at a specidic output pressure. If just presssing harder was an alternative why not go with a 1 inch booster and just "press harder" till ya get the stopping per of the 9 inch booster or the stopping power of the 11 inch booster? Hell why not just press harder till you got the stopping power of a 25 inch booster that certainly would stop my friends car faster in a short distance than mine stops with the puny 11 inch booseter.

See where I am going? How do you explain that theory? Boosters do not have a straight line "press more get more force" nature to them.

At some point (around (800 and 1100 pounds) these units are doing all they can do in terms of the function they provide for the brake system. I think that the boster is capable of producing just so much force. Afer it reaches its design limit that is all it is gonna produce. As backup to my thought I point the fact that GM used different size boosters and different dual dia boosters says it all. If the 9 booster was sufficient for ALL applications in ALL GM cars why are there 11 inch boosters and dual dia boosters? All GM had to do was universally use the 9 inch booster and put a tag on the brake pedal..."Press harder in this vehicle than you do in others...this vehicle equipped with a 9 inch booster.

I am gonna go to the gym now. This started as a discussion of why a 9 inch booster might not be sufficient for a disc brake equipped car. It has evolved into a vehicle dynamics analysis discussion that cannot be accomplished with the resources we have availble. In the real world sometimes we have to extrapolate from the data we have because we are never gonna have all the computational resoures to take all the factors cited in your post and that the car maginzines list as being part of the equiation in consideration.

Even if we did collect all the data you cite we don't have the analytic resources to actually calculate what should be happening and then compare that to what really happens.

I leave this discussion with the following words for the "just press harder proponents"...... PRESS ON GUYS

Geez Oman take a chill pill.

It is pressure at the pad/shoe that creates baking and the more you can create the more braking you get, right up to the point that the tire traction force is exceeded and you skid. And yeah we all agree that there IS a limit to how hard you can push the pedal. Once you reach that limit you will have to search elsewhere for more pressure if you need it.

The rub is pressure at the pads/shoes really is an interplay of a number of variables. And many/most of them can be related and figured out using simple math. It really is a summation of SEVERAL points of force multiplying leverage that gets you from pedal pressure to pad/shoe pressure. Applying some math can go a long way to explain and support experience. And to reduce the potential cost for improper or unsatisfactory parts.

It is not unheard of for parts to get mixed and matched on these cars. It is not difficult to create a poor performing overall system with just one mismatched part.

To the OPs point the size of boosters generally increased in later years when disc brakes came into vogue. I run a dual diaphragm booster from a wrecking yard '79 El Camino. It works as well as the large booster it replaced, even with my 11" of vacuum. And it clears the valve covers on my '68 BBC setup MUCH better. You can whack off the mounting brackets and pushrod from your current booster and adapt them to a newer/different booster. At the time I found web sites that detailed the conversion.

Wow you two act like your married. lol

I had a similair problem with the Baer 4 wheel disc setup I have on my car. I have the dual diaphram 8" booster. The kit came with a 15/16" steel bore master cylinder. I wanted a aluminum master cylinder and bought a 1" bore wilwood master cylinder. My brakes didnt stop very good. If I stood on the pedal they were a little better. I am also running a electric vacuum pump @ 18psi. At the time I could not find a 15/16" bore aluminum master cylinder.

So I put the steel one that came with the kit and my brakes worked fine. I ended up finally finding a aluminum 15/16" aluminum master cylinder and changed it and they work fine.

You are talking about different size boosters but piston size affects pressure. Bigger piston ,,less pressure,,more volume. Smaller piston,,more pressure, less volume. When I talked to the manufacture they said there was a 300psi difference in 15/16" bore to 1" bore. I figured 1/16" difference would not matter but it did.

Maybe check the bore size and go to a smaller bore size in the master cylinder.

Geez Oman take a chill pill.

It is pressure at the pad/shoe that creates baking and the more you can create the more braking you get, right up to the point that the tire traction force is exceeded and you skid. And yeah we all agree that there IS a limit to how hard you can push the pedal. Once you reach that limit you will have to search elsewhere for more pressure if you need it.

The rub is pressure at the pads/shoes really is an interplay of a number of variables. And many/most of them can be related and figured out using simple math. It really is a summation of SEVERAL points of force multiplying leverage that gets you from pedal pressure to pad/shoe pressure. Applying some math can go a long way to explain and support experience. And to reduce the potential cost for improper or unsatisfactory parts.

It is not unheard of for parts to get mixed and matched on these cars. It is not difficult to create a poor performing overall system with just one mismatched part.

To the OPs point the size of boosters generally increased in later years when disc brakes came into vogue. I run a dual diaphragm booster from a wrecking yard '79 El Camino. It works as well as the large booster it replaced, even with my 11" of vacuum. And it clears the valve covers on my '68 BBC setup MUCH better. You can whack off the mounting brackets and pushrod from your current booster and adapt them to a newer/different booster. At the time I found web sites that detailed the conversion.

I guess I am reacting to the extremes of the replies here. On one hand we have "Exercise you leg at the gym and press harder" on the other end we have what amounts to an engineering discertation on how many factors effect brake performance. The sublime to the rediculous.

I certainly plead guilty to reacting to the post before my previous post. The "How do you explain...." and the remainder of that post is just so far off base in terms of solving the original question that is just useless.

I agree it is easy to mismatch parts and get something that does not work. I think that is exactly what I am saying, and have been saying all along. Yes pressure at the pads is the result of an interplay of a bunch of parts. That said however nearly every part in the two systems I am speaking of is THE SAME save for the boosters.

That simple fact tells me something. I have difficulty with the polar opposite approaches to this problem that are presented here inspite of the similarities of the two cars. We are not talking about locking up the brakes. we are not talking about auto magazine tests, we are not talking about purpose built race cars.

We are talking about two street driven passenger cars that are stopping from the same speed using essentially the same equipment (EXACTLY the same equipment in terms of some of the hardware). The two cars demonstrate very different braking results. My days in engineering school and working in engineering lead me to ask.. "What is significant that is different between the two cars and what factors that indeed are different are insignificantly effecting this problem????"

All this about the race cars and the car magazines and all that is just so much fluff IMHO. What is the same...what is different between the two cars? That approach works for me. In point of fact the discs and calipers and pads and the master cyls are dead nuts compatible as are the porportioning valves.

Yes I am discounting / downplaying / minimizing some of the "different" parts like tires and wheel, sizes springs and what not. I think those differences are insignificant. They are insignificant at least until the major difference (the booster) is ruled out of the problem. Street tires are more or less street tires, steel rally wheels are steel rally wheels, the camshafts are more or less the same. A 65 Chevelle is more or less the same as a 72.

Short of swapping my booster onto my friends car and / or his booster on to my car and repeating the test what more can be done? That is not practicle because I am in Atlanta and he lives in Ct.

I like the keep it simple approach. The issue here is that a major component of the systems, namely the booster, is different between the two cars. That has to be the starting point for solving this problem, at least for me it has to be. Perhaps I am all wet, been wrong before and I will be wrong again. Until the booter variable is isolated and removed from the discussion I will maintain that the booster is the obvious starting point in seeking a solution to this problem.

Throwing all that other baloney into the discussion seemed like a waste of time to me and I spoke up.

I will offer a completely different solution. Take a look at hydratechs brake systems. It doesn't matter how much vacum you have. It was probably the single best investment I put on my 72. They are a little expensive, but worth every cent. My Chevelle now stops better than the wife's BMW.

I will offer a completely different solution. Take a look at hydratechs brake systems. It doesn't matter how much vacum you have. It was probably the single best investment I put on my 72. They are a little expensive, but worth every cent. My Chevelle now stops better than the wife's BMW.

This is not a vac issue. My buddies car has 15 inches. He tried some sort of booster augmenter, vac reservoir. I don't know what type. No effect.

I just put the Ground Up front disc conversion on my 66. It had four wheel manual drums with a single master. I did not get the booster, and apparently it has the same master as used with the booster. It still has the same old shoes and drums on back. I went this way because the engine has about 12 of vacuum, and it did not seem enough for a booster. I also just did not want the power stuff cluttering up under the hood.

It takes a firm foot to stop, but it stops straight and true, and for the first time in 3 years of ownership, it will lock the brakes!

It does not stop like a new car, but it can be driven normally now. No more filling the m/c each week, and no more creeping at lights with my foot on the floor.

I would look into stuff like the master cylinder diameter, smaller is better, and such. It should actually stop with no vacuum at all.

My days in engineering school and working in engineering

Maybe my being a high school dropout allows me to to see how simple it is to just push harder on the pedal? I still don't think the booster has anything to do with the braking distances of the 2 cars in question, but hey...what do I know?
Silly me, I bolted on an Aerospace manual disc kit to the front of my 69 Nova without changing a thing on the rest of the car, and it stops great from 137 mph--engineer types tell me it won't work without adding a disc/drum proportioning valve...I guess the car must be as dumb as me, because it works just fine.
Here's a link to the story I wrote for Hot Rod magazine:
http://hotrod.automotive.com/22812/40358-disc-brake-upgrade/index.html

Is the 9" booster dual diaphram?

For what little this may be worth I have a supercharged car that also sufferes from a lack of manifold vacuum. I have a stock 1972 Chevelle power front disc/rear drum system. Once the car became S/C'd I noted a degradation of stopping ability and in discussing this issue with friends someone suggested I try a set of ceramic pads. What a difference!

I gained back all that was lost and then some. I have 8" front and 10" rear tires and the car hauls down with authority. Unfortunately I have never tried successive stops to observe fade because in my day to day driving I don't encounter that situation. I am very satisfied with my car's ability to stop once quickly when I need it to.

Maybe that is the solution. Might be worth a try.

I posed the "canister" question to my slightly older but far better educated brother. Here's his analogy:

"This question succumbs to a logic argument. Let me use a city water storage tank as an example. Say City has a tank capable of storing 1,000 gallons mounted 100 feet above the town. Now City installs a larger 20,000 gallon tank mounted at the same 100 foot height. What is the pressure differential between the two tanks? Zero, because they are at the same height. What has changed is the volume of water available from storage. Or, looking at it from a disaster point of view, the town will get a lot wetter from 21,000 gallons of water flooding than 1,000, but it will be at the same pressure.

Now take a vacuum such as you describe. What is a vacuum? It is actually a partial pressure greater than zero (no molecular activity) but less than 1 atmosphere. The reference for engine vacuum systems is 1 atmosphere or 14.7 psia, the pressure we all experience at sea level at a reference temperature. The vacuum in an intake system really is just "less gas", not an actual vacuum. A true vacuum would be an enclosure entirely free of any gas molecules or, if not free of molecules, at a temperature of zero Kelvin. (gas in the sense of air)

The discussion you're involved in is the same as my water tank story above. By adding more volume at a partial pressure of 13 in Hg only storage ability has changed. There's more volume at 13 in Hg but the differential pressure between the power brake canister and your 55 gal drum is zero. The power brake storage canister plus the 55 gallon drum will mean the brakes will last lots longer without a running engine (ie more storage), but won't work a lick better because the entire system is at 13 in Hg. To reach 17 in Hg will require an auxiliary pump or less overlap.

To be more perfectly accurate you'd have to mount the 55 gal drum at the same average height as the power brake canister but for a gas like air the difference from roof mounting is microscopic."

I have the same issue with my '72. It was orig a 350, now a 502 ram jet. 10 to 11 inches of vacuum up to about 20 when decelating. My 275 lbs planted on the brake pedal and just slight pressure on the accelerator and the brakes lose. I notice that they are far less effective in slow speed traffic situations and need at least 3 feet to come to a complete stop after the pedal has done all it can to stop the car. Trying to run down the solution to this one - 11' booster, dual diaphram, hydralic brakes? I have noted that some of the upgrade systems for complete brake replacements ask what was there to begin with drum/drum, disc/drum or disc/disc - is there a difference in stock parts that will influence how the care brakes on conversion? Thoughts folks please - struggling to resolve this issue as well.:thumbsup:

(I know most of this thread is from 2009)

The differences here come down to leverage vs power.

Any brake system - drum or disc, rod or cable or hydraulic, power or manual - is a lever system, converting the work (force applied over a distance) of pressing the brake pedal several inches with a force of x pounds into the same amount of work at the brake unit (caliper or slave cylinder), but with a much greater force (thousands of pounds) acting over a much smaller distance (~1/16th of an inch). It's exactly the same principle as the jack you use to raise you car: you pump your jack through an arc of about four feet about ten times (= forty feet) to raise the car one foot.

If you know the length of your brake pedal and the diameters of the various cylinders, you can calculate the total leverage in the system just like you calculate the total gearing from the engine to the rear wheels using the transmission and rear end ratios and the tire diameter. It's very simple math, and it's an absolute number.

As this is a simple leverage equation, you can see that a given amount of work (stopping power) can be obtained by different ratios of force to distance (W = F x D). If you use a higher ratio, you will be able to use less force, but will need to exert it over a longer distance. This is the trade off in brake design - how far do you want the brake pedal to move between "no brakes" and "Oh SH|T!"? If it moves too far, it may be too far to swing your leg in the event of an emergency, or just too far to "feel right." There's also only so far you can get the pedal to move. If it moves less distance, you will need to push it harder to get the same amount of a stop.

By varying cylinder sizes, you can make the pedal easier to push, but if you miscalculate, you'll hit the floorboards before you've achieved maximum braking force, which would be bad, or you'll have to start with the pedal up by the steering column, and your knee against your chin, which gets annoying.

The vacuum (or hydraulic) booster adds another dimension to this equation.
The booster actually ADDS FORCE, like somebody putting their foot down on top of yours.
It does this by making the manifold vacuum do the work of moving the booster diaphragm, and adding that work to your own. Therefore, you can get more braking without increasing the pedal travel, which opens up a whole world of new design options for automakers.

So, yes, you can stop the car without a booster, and if the amount of force required is comfortable for you and the pedal travel distance is reasonable, that's a reasonable option. BUT, you can also increase the force applied, without changing the travel, by adding a booster.

It's really your choice, but you need to understand what you're choosing, and what the limitations are of what it will do.

- Eric

So gentlemen, if you were re-doing your chevelle brakes would you buy the 9" or the 11" booster? Thanks.

All other things being equal the 11" will do a better job than the 9" given a constant vacuum supply.

If you don't mind fiddling with master cylinders or pedal travel a smaller diameter cylinder will provide more output pressure for a given input pressure but it will require a longer pedal travel to do it.

That said, the diameters available are likely fairly narrowly incremental as would be pedal travel changes. Then there's always the ability to relocate the pushrod...

I don't know anything about the theoretical analysis provided but I do know this:
I put a new 9" booster from G"U on my car when I was restoring it.
Brakes worked ok but had to stand on them and still didn't stop well.
While bleeding the brakes I bent the pushrod from the booster to the pedal.
Bought an 11" booster from YearOne, they are withing driving distance-2 hours, and gothome and put it on.
Totally different brakes. Much much better.