Thread: Brake troubles

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Only a guess here, Maybe the rear disc calipers are not the same size as the front calipers? I beliieve your master cylinder (assuming this is a disc MC and not a drum as you have equal sized reseivors) only can and will move a given amount of fluid to the front and rear, your metering valve and your porportioning valve limit pressure or hold off the timing not volume, if it was to limit volume the fluid would have to be displaced somewhere and its not as the pedal travel remains the same. So if your front calipers are a Wilwood or small volume caliper and your rears are a GM metric or corvette as an example you will never move enough fluid to collapse the GM's as the wilwoods collapse with less volume and once collapsed the pedal can no longer travel anymore to move more volume. This is only true on disc setups as a drum master cylinder moves a different volume for the front and rear. Sorry for the long answer and I hope I explained it enough to either solve or eliminate this as the problem, but I could also tell you how I know this but I am sure you know why! Best of luck

Originally Posted by Matthyj

Only a guess here, Maybe the rear disc calipers are not the same size as the front calipers? I beliieve your master cylinder (assuming this is a disc MC and not a drum as you have equal sized reseivors) only can and will move a given amount of fluid to the front and rear, your metering valve and your porportioning valve limit pressure or hold off the timing not volume, if it was to limit volume the fluid would have to be displaced somewhere and its not as the pedal travel remains the same. So if your front calipers are a Wilwood or small volume caliper and your rears are a GM metric or corvette as an example you will never move enough fluid to collapse the GM's as the wilwoods collapse with less volume and once collapsed the pedal can no longer travel anymore to move more volume. This is only true on disc setups as a drum master cylinder moves a different volume for the front and rear. Sorry for the long answer and I hope I explained it enough to either solve or eliminate this as the problem, but I could also tell you how I know this but I am sure you know why! Best of luck

You may have the answer to my problem, not sure. The calipers on the rear are 1978 to 1988 GM.
The fronts are also GM but maybe different volume is used to operate.
Matthyj: what was your fix ?