As long as there are no leaks and the brake hoses are not
swelling under pressure it is likely the bypass valve in the hydraulic control unit is leaking. Common when a brake job is done and the calipers are compressed without opening the bleeder screw.
Easiest way to check the master cyl. is to remove the lines and thread in plugs to block the ports. Then step on the pedal and if its hard and not fading you know the master is ok. If the rear brake shoes are wore out or not adjusted correctly you will have a low/spongy pedal
I have seen the RWAL valve cause this problem several times. You can still buy them through aftermarket parts suppliers since Ford discontinued it.
Also have a assistant check the firewall for flexing when the pedal is applied. It is not uncommon for the firewall to crack where the booster mounts and then the firewall flexes.
I also remember having to change the master cylinder to one with a larger bore to correct the problem you are describing. I believe there is a service bulletin on that with part #'s
SOURCE: bad brakes on gmc suburban,
Gravity bleed your brakes starting at the wheel farthest from the master cylinder. No pweddle pumping. I Fill M/C and leave cover off. Then open the bleeder and watch what is coming out. Uou can see bubbles. When the fluid drips steady and without bubbles, cole and do the same to the other three. Don't let the master cykinder get low because thr fluid level helps move the fluid and air through the lines.Periodically tap the break lone with your wrench to keep everything moving.. When sll bleeders drip clean and steady. Make sore your rear brakes are adjusted to barely touch the drum. Then top off the master cylinder, cap it and give yourself a "break". :)
SOURCE: i have a 92 chrsy. 5th ave with no antilock
If the pedal drops to the floor without you stepping on it or with very little effort then i would say bad booster or bad check valve on the vacuum line going to the booster. Hops this helps.
SOURCE: 89 c1500, full pedal on brakes until started then
adjust your rear shoes up so that the rear drums just turn, with them ajusted to a low setting the brake pedal will be low .
also when you start your engine it is noral for the pedal to drop because of the boster ! [ but not to the floor ]
Bruce Pike
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