This is a 69 Chevelle Malibu. I was told it was a 4 wheel drum, manual brake car. Someone (not us) has put a delco (gold anodized) booster and master cylinder on it. Now the vehicle came in to our shop with an over brake problem. He said the brakes were great before (why fix what isn't broke.... i know..?) but after putting the booster and master cylinder on, the vehicle has a terrible over brake.

So we did the obvious stuff first, made sure all the lines were ran correctly, making sure all the shoes were on right... wheel cylinders proper bore... etc.

Then we started throwing parts at it, so far I have gotten the 4 wheel drum booster and master cylinder for a 69 chevelle malibu and even with that set up that over brake is still there. We blocked off the rear brakes, and the fronts seemed to improve slightly, we blocked off the front brakes and it made no difference.

(by overbrake I mean the split second you barely touch the brake pedal the wheels lock up, front first)

So the questions are:

Is there a different proportioning/combination/distribution valve/block set up between a manual car and a power brake car?

If a power brake car and a manual brake car use the same block/valve, what am I missing?

Thanks guys

Steve,you might check and see if they installed the rod from the booster in the correct hole on the brake pedal. It should be in the lower position for power brakes. I would also remove the pin that connects the booster rod to the pedal and make sure there is no preload adjusted into the brakes.
Mike

Thank you for your quick reply.

I got with the technician this morning and he confirmed that it was in the lower hole, and there was no preload, but the problem still exists.

Thanks again.

Is there a different proportioning/combination/distribution valve/block set up between a manual car and a power brake car?
The car still has drums front and rear, yes?

There is a difference between drum / drum and disc / drum combination valves since discs do not require residual pressure in the lines like drums do. Did they switch distribution blocks when they did the booster / mc? Maybe they got the wrong one?

The vehicle is still a 4 wheel drum car. The distribution block is the same, and there are no other delay/proportioning valves in the system.

Steve with the car on the rack do you find any of the brakes dragging?
Mike

No drag. And the problem still occurs if we grossly adjust the brakes in.

If they used a smaller dia. master cyl., it will increase the pressure, PSI, to the system than a larger dia. MC. Are you sure the wheel cylinders aren't oozing enough to cause lock-up? One on each end could cause this too. Any obvious changes to the carb or vacuum source?

Steve have you guys got a brake pressure gauge you could hook up and see what kind of pressure you find at the wheel cylinders,front and rear?
Mike

The shoes and wheel cylinders are great on it. He even roughed the shoes up a little to check for glaze or whatever.

I think we're coming done to a pressure problem. I had assumed he had verified pressure already, but he has not. I'm hoping to get results from that soon. The masters we've been using are 1", I may try to find a 1.25".

(by overbrake I mean the split second you barely touch the brake pedal the wheels lock up, front first)
How are the tires? Air pressure? Old, hard rubber? Tire size and wheel width seem appropriate?

How are the shock absorbers?

Front wheel bearings adjusted properly?

Are the wear surfaces of the drums good? Smooth, no heat-checks, no grease, no cracks, no bellmouth?

When you just barely push on the brake pedal, do you see a small "geyser" in each brake fluid reservoir?

Obviously everything can't be right for there to be a problem. But the brakes, tires, wheels, shocks and everything is in good working/clean order.

We put a smaller booster on today and we seem to have more brake control. Without a pressure gauge to see how much pressure we're getting we're just guessing.

When I added an 11" booster to my (otherwise stock) 4-wheel-drum camaro, it made the brakes very touchy. I just dealt with it until I converted over to discs up front, but it always seemed like an issue of "too much" booster and losing pedal feel. If changing to a smaller booster (less assist) helped, then you should also be able to improve things with a larger diameter master cyl (I'd probably try a 1+1/8" bore setup). I think some other folks had mentioned something similar, but I can't remember if it was here or on the Camaro site.

Update:

Using a smaller booster and an 1.125 master, the braking improved some.

Finally we were able to get our hands on a brake pressure tester. Getting 900 psi at the rear brakes. Went to Jegs and ordered 2 Wilwood brake pressure limiters. See if we can't get the pressure down to 500-600 and get this thing out of the shop.

Steve,thanks for the update. Are you sure the owner didn't plumb the front brake side of the master cylinder into the rear brake side of the combination valve?
Mike

I'm sure it's plumbed correctly. A drum brake car (at least this one) doesn't have a combination valve, only a distribution block (with a valve to turn on the dummy light if pressure is lost). There are no other delay valves or proportioning valves in the system.

Still waiting for Fed-Ex to bring us the limiters, fingers crossed.

Getting 900 psi at the rear brakes. Went to Jegs and ordered 2 Wilwood brake pressure limiters. See if we can't get the pressure down to 500-600 and get this thing out of the shop.

1. If GM didn't need to install pressure limiters on drum/drum vehicles--or a pressure limiter on the front brakes of disc/drum vehicles...why do you? Something else is still wrong, you just haven't found it yet.

2. The amount of pressure generated--which you say is "900 psi" is purely dependent on how hard you push the pedal. Getting the pressure down is as easy as "Don't push so hard!". What are your pressure limiters going to do when the customer panic-brakes?

1. If GM didn't need to install pressure limiters on drum/drum vehicles--or a pressure limiter on the front brakes of disc/drum vehicles...why do you? Something else is still wrong, you just haven't found it yet.

2. The amount of pressure generated--which you say is "900 psi" is purely dependent on how hard you push the pedal. Getting the pressure down is as easy as "Don't push so hard!". What are your pressure limiters going to do when the customer panic-brakes?

I understand what you're saying. Everything else on the brake system checks out. There was nothing 'wrong' with the brakes to begin with, he just wanted the assist of a booster. The moment he added a booster he's had an overbrake/over sensitive brake issue. It's not like there was another problem and the booster amplified it, or something else was wrong to begin with. He's upset because the brakes are a little grabby or brakes too easy, the second you lay half a toe on the pedal you're eating windshield.

The pressure limiter doesn't restrict pressure to a certain limit, it takes the edge off of it. During a panic stop they will have higher pressure for that panic stop. I'll just be able to take 100 psi at a time off of it until the pedal feels good again.

Again I appreciate any and all feedback and suggestions, but you haven't really brought anything to the table. If you have any suggestions or ideas that haven't been covered here already, let us know. Thanks.

you haven't really brought anything to the table. If you have any suggestions or ideas that haven't been covered here already, let us know. Thanks.
Tried that earlier, didn't get my questions answered.


When you just barely push on the brake pedal, do you see a small "geyser" in each brake fluid reservoir?

GM sold a bunch of vehicles with drum/drum brakes and a power booster; and they did that across many model years and plenty of body styles. There's no inherent problem with that set-up that requires pressure limiters.

Then we started throwing parts at it
I understand you're frustrated, but there's got to be SOMETHING you're missing.

Steve you might want to pull the M/C off and check the depth of the hole in the piston against the distance that the pushrod extends out of the booster past the mounting flange for the M/C. Apparently some applications use a shorter or longer pushrod. I know you said you tried another booster,but it might be worth taking the time to measure it.

From this site: http://www.thebrakeman.com/valvetechi
Occasionally, another problem in the brake system will be mistaken for a residual valve. This is when the master cylinder pushrod length has been improperly adjusted in such a way, as to not allow the piston to return all the way to the back of the bore. If the piston does not return all the way, the cup in front of the piston will not move back far enough to open the bleed hole. When the bleed hole is not open, residual line pressure is the result. Unfortunately, this type of residual pressure is not controllable and gets worse the harder the brakes are applied. Master cylinder pushrods always should have "free play" to insure that the piston can return all the way back when pressure is released.


From this site: http://www2.yearone.com/serverfiles/fbshopmain2.asp?hid=018AC48152&Style=2
New cadmium-plated power brake booster. This booster comes with long and short pushrods and features the correct "Delco Moraine" stamp as original.

Mike

Could the brake shoes have been contaminated at one time?
Thet might look clean but if they were grease/oil soaked or brake fluid soaked they will grab like crazy, especially when they're cold.
It might be worth trying a new set of shoes.
Maybe look at the hardware also. The return springs might be weak

Tried that earlier, didn't get my questions answered.

I answered all but the geyser question. That I can't confirm or deny at this time, for ignorance sake, what would that prove? Not trying to be a smart ass, just asking (it's easier to convince a technician who 'knows more' than me if I have a good argument for doing it, lol).

GM sold a bunch of vehicles with drum/drum brakes and a power booster; and they did that across many model years and plenty of body styles. There's no inherent problem with that set-up that requires pressure limiters.

I bought the original power booster and master cylinder for a 69 Chevelle Malibu.

I understand you're frustrated, but there's got to be SOMETHING you're missing.

Yes, I know there's something we're missing, that's why I'm here seeking the infinite knowledge of the cumulative genius here at chevelles.com :).

Mike,

My technician swears that pedal rod length is correct. He confirmed there's 1/8" of distance between the rod and the master cylinder at rest. We've used 3 different style boosters, and 4 different master cylinders. (different bores/lengths etc.)




Update: Brake pressure limiters do cause it to brake properly. Now we have to debate on trying to resolve that, or leave them in and call it a day. Probably going to be up to the customer at this point.

Steve you might want to pull the M/C off and check the depth of the hole in the piston against the distance that the pushrod extends out of the booster past the mounting flange for the M/C. Apparently some applications use a shorter or longer pushrod. I know you said you tried another booster,but it might be worth taking the time to measure it.

From this site: http://www.thebrakeman.com/valvetechi
Occasionally, another problem in the brake system will be mistaken for a residual valve. This is when the master cylinder pushrod length has been improperly adjusted in such a way, as to not allow the piston to return all the way to the back of the bore. If the piston does not return all the way, the cup in front of the piston will not move back far enough to open the bleed hole. When the bleed hole is not open, residual line pressure is the result. Unfortunately, this type of residual pressure is not controllable and gets worse the harder the brakes are applied. Master cylinder pushrods always should have "free play" to insure that the piston can return all the way back when pressure is released.


From this site: http://www2.yearone.com/serverfiles/fbshopmain2.asp?hid=018AC48152&Style=2
New cadmium-plated power brake booster. This booster comes with long and short pushrods and features the correct "Delco Moraine" stamp as original.

Mike

Mike,

My technician swears that pedal rod length, etc, are correct.




Update: Brake pressure limiters do cause it to brake properly. No we have to debate on trying to resolve that, or leave them in and call it a day. Probably going to be up to the customer at this point.


Take a look at what M.Maner is talking about. There is a rod between the booster and MC. Not the rod going to the pedal, they are different. When I installed my booster my booster rod was ~1/4" to long (using the short rod) and I had to grind a little over 1/4" off to get it to work properly. It causes all sorts of issue's. I've had this problem with both my cars. It's worth a quick look.

Good luck.:thumbsup:

Okay got with the technician. He said there's no preload on it at all. With the pressure gauge on the system it drops down to 0 pressure at rest.

He also confirmed that he had already modified the short rod to see if it would help, it did not.

I think we're just gonna leave the limiters on and call it a day (or two weeks... /sigh lol).

Thanks for all your help guys.

Checking for two "geysers" in the reservoirs will POSITIVELY VERIFY that the pushrods aren't too long; and it takes about thirty seconds--less if you already have the master cylinder cover off already.

The whole point of checking for the geysers is to accomplish the same thing as M.Maner wanted you to do--check the pushrod lengths. My way is a lot faster and easier.

The "geysers" are activated by the primary and secondary pistons moving forward from their at-rest position BEHIND the compensating ports of the master cylinder. The pistons move forward, but until the seals are past the compensating ports--fluid gets pushed back into the master cylinder. Thus the geysers.

If the pistons are not "at rest" BEHIND the compensating ports, at least one of the two pushrods has to be too long/misadjusted--or--the wrong master/booster combination is being used.

It's getting so that looking for the geysers is one of the first things I do to diagnose brake problems.

Checking for two "geysers" in the reservoirs will POSITIVELY VERIFY that the pushrods aren't too long; and it takes about thirty seconds--less if you already have the master cylinder cover off already.

The whole point of checking for the geysers is to accomplish the same thing as M.Maner wanted you to do--check the pushrod lengths. My way is a lot faster and easier.

The "geysers" are activated by the primary and secondary pistons moving forward from their at-rest position BEHIND the compensating ports of the master cylinder. The pistons move forward, but until the seals are past the compensating ports--fluid gets pushed back into the master cylinder. Thus the geysers.

If the pistons are not "at rest" BEHIND the compensating ports, at least one of the two pushrods has to be too long/misadjusted--or--the wrong master/booster combination is being used.

It's getting so that looking for the geysers is one of the first things I do to diagnose brake problems.


Yep, Schurkey is right on the money as well... Although thinking about it a little more I would think overbraking and locking up the wheels (all 4) that easy would be that you are moving WAY to much fluid to the slave cylinders too fast. My thought would be master or slave cylinders (one of the 2) are not correct.

Thanks for the explanation Schurkey,very interesting. If the internal "leak" back into the M/C were eliminated. As in the piston to far down the bore,you would in effect be moving more fluid. If the pedal stroke begins with fluid being pushed back into the master cylinder rather than being forced down the brake line, you have pedal travel without creating full brake pressure.
Mike

Thanks for the explanation Schurkey,very interesting. If the internal "leak" back into the M/C were eliminated. As in the piston to far down the bore,you would in effect be moving more fluid. If the pedal stroke begins with fluid being pushed back into the master cylinder rather than being forced down the brake line, you have pedal travel without creating full brake pressure.
Mike
True from a certain perspective. Having the piston "at rest" in the proper location--so that the initial movement of the brake pedal pushes fluid back into the reservoir--would primarily do nothing more than increase the pedal travel a slight amount.

If the piston does NOT rest to the rear of the compensating ports, pedal travel is reduced slightly; and fluid is directed to the wheel cylinders instantly upon brake pedal movement--but the stroke of the piston is also reduced a bit, too.

The "big problem" with having the pistons "at rest" ahead of the compensating port is that the brakes can "self-apply". As the fluid in the wheel cylinders warms up and expands--it builds pressure in the hydraulic system instead of venting to the reservoir. As the pressure builds--the brakes apply more firmly, creating more heat, expanding the fluid even more, which applies the brakes more firmly still--and in the most extreme cases, the cycle continues until the smoke comes out of the wheelwells, and the car doesn't have enough horsepower to move any more. Once everything cools off--the vehicle drives away again.

and fluid is directed to the wheel cylinders instantly upon brake pedal movement--


This is what I was suggesting in the previous post as a possible cause for the brake problem. All very interesting Schurkey,thanks.
Mike

Checking for two "geysers" in the reservoirs will POSITIVELY VERIFY that the pushrods aren't too long; and it takes about thirty seconds--less if you already have the master cylinder cover off already.

The whole point of checking for the geysers is to accomplish the same thing as M.Maner wanted you to do--check the pushrod lengths. My way is a lot faster and easier.

The "geysers" are activated by the primary and secondary pistons moving forward from their at-rest position BEHIND the compensating ports of the master cylinder. The pistons move forward, but until the seals are past the compensating ports--fluid gets pushed back into the master cylinder. Thus the geysers.

If the pistons are not "at rest" BEHIND the compensating ports, at least one of the two pushrods has to be too long/misadjusted--or--the wrong master/booster combination is being used.

It's getting so that looking for the geysers is one of the first things I do to diagnose brake problems.

A very informative post. I feel less ignorant just reading :).

That makes a lot of sense to me as a good way to check. I'll have the tech do it tomorrow just for good measure.

We have confirmed two "geysers" in both sections of the reservoir.

I'm all out of ideas. If you guys figure it out post back and let us know what you found.
Mike

THIS guy solved a brake lockup issue by replacing contaminated shoes. It's not clear how obvious the contamination was. He says he didn't smell brake fluid on the shoes; but he doesn't say anything about how they LOOKED.

http://www.chevelles.com/forums/showthread.php?t=282618