I've a 92 with a hard pedal problem, and its a bit hard to stop. Its got the big brake package, low miles 42k car, newer GM rotors and pads. Noticing a hard pedal over the last couple of years, ( its a convertible and not used often), I thought maybe the pads were glazed a bit, so didn't worry too much till it struck me, and I checked out and found the pedal was just as hard when the engine was off and it had no vacuum, as when the engine was running with 16" vac. So after installing a new power booster, as manual indicated it needed, SAME results. I checked pedal travel with the engine off, and the vac bled off, its only 1.125"
at 70lbs foot pressure, where the manual calls for no MORE than 2.75". That indicates to me maybe its not right yet, so any ideas where to check next? About the only thing I can think of is the master cylinder, but what could be wrong with it? I've bled brakes twice since the car was new, the second time when I added the GS calipers and ZR1 rotors, so I'm pretty sure its not had contaminated fluid and the swollen seals that go with that, besides, the brakes do not drag. Any ideas are truly appreciated. Thanks in advance

 

Sounds like the classic symptoms of a brake booster that has gone south. Check it out and if you need to replace, consider the tin/metal one and not the black plastic one.

Good luck.

 

I've heard/read this before, so this very likely could be the problem. Question: I don't recall the reasoning; plz explain again what it is about the plastic jobbies that don't hold up vs. the tin/metal ones. Thanks.

 

All that has been said is true and it sound like you have the classic booster problems. A metal one might be better, but there’s no reason that a properly operating plastic one should not work.

Thoughts and things to check:
- Defective new booster
- Does the booster hose have plenty of vacuum, place your thumb on the hose, it should suck it pretty good and when removed, the engine should speed up.
- Check valve in booster not working right. You can (use alternate hose) suck on the valve with you mouth and you should be able to draw easily.
- Does you idle change when you step on the brake and is there a whoosh sound.
- Remove the bolts from the master and push back a bit. Push pedal 3 times and on the 4th there should be some resistance to travel. It should move several inches freely but with some resistance. Now start the car and push on the pedal . It should almost float to full travel, this is a normal condition. If it does not, the booster may be not working.

I have used these measurements in the past and even though you pass the test I have concluded that it is almost useless information. If it ever moved 2.75 inches, you have other problems like a master, caliper or air. Nice info but so what. Your measurement sounds reasonable to me, the problem you have is when you start the engine and there is vacuum, the pedal should respond differently due the booster coming alive.

I would not think you have a clogged flex line or ABS system since you have not indicated a puling of the brakes. A defective master would be unlikely given the current facts. Before moving forward into the rest of the system, you have to determine if in fact the booster working. If it’s bad replace it, if it’s vacuum fix it.