I have a 1975 Corvette with a brake pedal that goes to the floor but will stop the car. This problem has been with the car for quite a while but the car is not driven and sits in the garage. Some time ago I replaced the master cylinder but that didn't solve the problem. I recently replaced all 4 brake calipers and used a vacuum pump to bleed the brakes. The pedal still goes to the floor. With the engine off and the master cylinder cover off I can press the pedal about 3/4" and see fluid movement in the master cylinder. With the engine off and pressure on the pedal, the pedal goes about half way down. With pressure still on the pedal, starting the engine causes the pedal to go to the floor. I read this is a way to check the power brake booster and if the pedal goes down the booster is good. Should I try another master cylinder? I hope the booster doesn't have to be changed because it looks like a big job (pulling the dash etc). Thanks for any help you may provide.

 

I HAD THE SAME PROBLEM WITH MY 82 , I DON'T KNOW HOW YOU BLED THE BRAKES WITH A VACUUM PUMP,BUT I DID MINE THE BLEEDER WAY,THE SEQUENCE OF BLEEDING IS VERY IMPORTANT.I DIDN'T THINK SO AT FIRST BUT AFTER BLEEDING THEM 3 TIMES , I FINALLY GOT THE CORRECT SEQUENCE FROM A POST ON THE FORUM,I DON'T KNOW THAT YOURE CAR IS THE SAME AS MINE BUT THAT DID IT FOR ME. YOU STILL HAVE AIR IN THE SYSTEM,I BELIEVE THE INFO I GOT WAS ON A AUTO ZONE WEB PAGE.HOPE THAT HELPS

 

I'll bet you have air in the line.

 

Be careful here. There is definately air in the line. Look for leaks or a rotted brake line. The air got there somehow. If you have a rotted brake line, the proportioning valve has a neat little safety feature. It has a slide valve in it that will block off half the brake system if you stop the car with a fluid leak. So, if the rear line is rotted and leaking, the valve will block off the rear brakes and only use the front brakes to stop the car. And vise-versa for the front brakes. Usually you have to step on the brake pretty hard to make the slide valve move. Once it does, you have to take off the low pressure sensor from the proportioning valve, and slide the valve back to the center by hand with a screwdriver. Then you can bleed the brakes properly. I just had a car in here that that was the problem. The female owner didn't know she only had front brakes. ....until one of those lines rotted out too! Turns out, she had been driving with only front brakes for a looooong time. The rear brakes were siezed in place and took some doing to free up. I wound up having to replace every steel line on the car because they were all rotted. What wasn't rotted through, broke when I tried to remove a line next it.

Don't take any chances with your brakes. Inspect every piece carefully. Take things apart and really look at stuff. I'm willing to bet there's a rotted line somewhere in that car. It happens alot to cars that sit. Even if they're in a garage.