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Brakes are too weak. Suspect master cylinder.

573 Views 7 Replies 5 Participants Last post by  Eduramac
I'm having some braking issues on my 77 Corvette. It has power brakes. The master appears to be the original unit, the proportioning valve is much newer. The pedal is not soft, it feels about the right firmness and goes about 75% to the floor max. But the brakes are just too weak. The car stops too slowly. It's impossible to lock up the wheels and skid no matter how hard you press the pedal.

No leaks. All lines are in great shape - no rust. The sensor on the proportioning valve is not grounded (meaning the fluid flows to the front AND the rear).

I cracked the bleeder valve open on the front passenger side caliper, hooked up the whole clear hose & bottle set up and tried bleeding it. The first 3 or 4 pumps, the fluid came out briskly and quickly filled up the clear hose. But after that it slowed to a trickle and every subsequent pump on the pedal only forced a tiny amount of braking fluid out. I think that's the issue.

I also checked to see if the sensor on the proportioning valve goes off as I press the brake pedal (suggesting it restricts flow to the front), but it doesn't.

I'm out of ideas. What could be the issue? Master cylinder?
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Do they work better on a 2nd pump on the pedal while trying to stop the car?
Reason why i ask is because this would indicate air in the lines, (usually the back) and bleeding the whole system would help.
Also, is it disc or drum in the rear? If drum, they could probably use adjusting and this will increase your stopping power greatly.
 

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It's disk brakes all around. Also, the brakes actually work better on the first pump after driving without braking for a while. After that first pump, they get pretty weak. I wonder if this has something to do with the vacuum booster. The motor has a very aggressive cam in it and the vacuum at idle is literally like negative 6 psi. So it probably takes a little while to build up the vacuum in the booster, but once you brake, it doesn't replenish very quickly and if I'm braking again and again in close succession, the brakes just don't work very well. Maybe I should get a vacuum canister. But then again, even with weak vacuum the brakes should still work fine just with a little more work by the foot. Really not sure what's doing on.
Without help from the booster, the pedal will be rock hard and nearly impossible to stop the car at anything over 10 mph. Even with both feet pushing very hard.
 
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