My brake master cylinder started leaking so I replaced it. While I was at it I also replaced the lines with new stainless steel lines and replaced the front calipers (leaking too). Looks killer! Since we all like pictures: http://i245.photobucket.com/albums/gg80/nwanklyn/DSC04356.jpg

Here's my problem...THEY ARE REALLY TOUCHY! Scary touchy actually, it'll throw you through the windshild with a toe. :confused: Is there any way to fix this? Did I do something wrong? Are they suppose to be this touchy?

Help...Thanks,
Nick

Sounds like you got the wrong size MC which is makin them too touchy. You need to check the old MC bore and compare with the new one. I bet they gave you a different on from your original.

Sounds like you got the wrong size MC which is makin them too touchy. You need to check the old MC bore and compare with the new one. I bet they gave you a different on from your original.

The new master cylinder is physically taller than the old one holding a larger volume of fluid, so that makes sense. Can I pop it off the booster and measure it there or is it internal to the master cylinder? I can't measure the old one because it was my core. Anyone know what the correct bore should be?

Amout of fluid volume is not going to make any difference here, it has to do with the bore of the MC. That will determine how much pressure is applied, not sure what the correct size is for your car. Disc? Drum? Power Disc?

Amout of fluid volume is not going to make any difference here, it has to do with the bore of the MC. That will determine how much pressure is applied, not sure what the correct size is for your car. Disc? Drum? Power Disc?

I realize the fluid capacity is not the cause, but a bigger bore will need more fluid...right? Therefore a larger resevoir? Just trying to put 2 and 2 together. I think I agree with the too large bore diagnosis, just want to make sure I don't get the wrong one again. Anyone know the correct bore? Guess I could calculated it.

Sorry, it is power with front disk, rear drum. Very stock setup.

Thanks for the help :thumbsup:

Did you mess with the rear brakes at all? Rebuild or adjust them? Does this happen all the time, or just when you first use them? Reason I ask is, my wife's Durango with drums on the rear will just about throw you through the windshield too first thing in the morning, because the drum brake shoes are cold. It's always been that way. I move 50' from my driveway to the corner, and forget every time in the morning, because she normally drives it. Once it's warmed up just a bit, it's fine.

As far as bore size, actually, a larger bore, while moving more fluid, will have less line pressure. One thing you might want to check is that the rod from the booster isn't pushing on the master at all while at rest. Pull the master cylinder off the booster, and measure the length of the rod from the booster while at rest, and measure the depth of the pocket in the master. Measure both from their respective mounting flanges.

The only thing I did to the rears was bleed them, no adjustment. The brakes are touchy all the time, nothing makes a difference.

I'll check the length of the rod vs. the depth of the pocket. I know they are not engaged with no pressure on the pedal because I checked the wheels to for drag already. Mayby there is a gap and it engages suddenly.

As far as the line pressure, it will be the same actually, just means it takes more input force (your foot) to move the same volume of fluid. Hydrolic brakes work by displacing volumes of fluid. Having a larger bore means less travel to move the same volume. The larger surface area makes it easier to move this volume. These two combined makes for touchy brakes...right?

Anyone know what the correct master cylinder bore should be? Or maybe my theory is incorrect.

Thanks

What Cameano stated but wording it differently--- The larger bore master cylinder will require more pedal effort to achieve the same line pressure.

"I realize the fluid capacity is not the cause, but a bigger bore will need more fluid...right? "
That is backwards, it is a leverage deal, smaller makes more line pressure. The following is right in theory but number are just for illistration, lets say the goal 25LBS pedal pressure puts a 1,000 lbs pressure on the bake pads. Pedal has say a 4 to one ratio so we put 100 lbs on the MC piston, than say the ratio of the area of a 1.25" MC to the Area of the combined caliper or wheel cylinder pistons is 10 to one. All is good, now parts guy sells you a MC with a 1" piston, run the numbers and you will see why your brakes are touchy

"I realize the fluid capacity is not the cause, but a bigger bore will need more fluid...right? "
That is backwards, it is a leverage deal, smaller makes more line pressure. The following is right in theory but number are just for illistration, lets say the goal 25LBS pedal pressure puts a 1,000 lbs pressure on the bake pads. Pedal has say a 4 to one ratio so we put 100 lbs on the MC piston, than say the ratio of the area of a 1.25" MC to the Area of the combined caliper or wheel cylinder pistons is 10 to one. All is good, now parts guy sells you a MC with a 1" piston, run the numbers and you will see why your brakes are touchy

A bigger bore does not need more fluid. Brake fluid for practical purposes is not compressible. The amount of fluid volume needed is dependent on volume need to move the calipers/drums from the relaxed non applied position to the applied position.

For the same applied effort the smaller bored cylinder will creat a greater PSI pressure and will have a longer travel.

If you apply 10# to 1 sq inch of area you have 10 PSI. If you apply 10# to 2 square inch of area you now have 5 PSI.

Maybe prop valve by MC not working? Should hold off front discs for a second.

I need to pirate this thread !

2 weeks ago I replaced "just the booster" on my 72 rag,
all drum brakes, and it now wants to put me also thru the windshield,
no need to discuss pressures, or bore sizes, or leverages,
all stayed the same,
is there a valve that should be in the vacuum line to reduce
source vacuum to the booster ?

Thanks
Rick

Nick, have you found your problem yet ?
Thanks
Rick

Nick, have you found your problem yet ?
Thanks
Rick

No I haven't. Thanks to this thread I think I've ruled out the wrong bore size as my problem. The car sat for quite a while so maybe the propotioning valve or hold-off valve is to blame.

Also, I noticed the pedal arm had 2 holes for the brake rod to attach. Mine is in the top hole giving me more leverage. I couldn't get it to go to the bottom one, looks like I need a longer rod. I'll try this next, then move on to the proportioning valve/hold-off valve.

Thanks for all the insight! Hopefully I'm on the right track.

Nick

Just doing the brakes on my 70 with power disc front and drum rear. My MC has a 1 1/8 bore.
The old prop valve was full of crud but had still been working last season. Prop valves are available for 70's from Inline Tube or Ground Up.

I 'm just thinking out load here but I seem to remember that there are two different length rods that go between the booster and the rear of the master cyl.. gary

I hated the way my power brakes worked. Just like you described and would lock up real easy. Once you get use to the feel it gets better. But the best mod I did was replace the master cylinder with a manual master brake cylinder from a 68 camaro and get the right rod. It does away with the booster and that crazy grabby feel. best mod I've done as the brakes are like a normal car now and don't worry about vaccuum.

Replaced the front pads and had the drums turned,
Much much smoother, especially when cold
Rick