While driving, brake light came on, brake sunk straight to floor. Noted back reservoir MC empty, assumed rear brake line leaking

checked brake lines, Noted some brake fluid on the proportioning valve reset. Appears to be seeping. Replaced proportioning valve. No further known leaks.

bled brakes in proper sequence. Car off. Brake pedal firm. Turn car on, brake pedal again goes straight to floor.

checked booster by disconnecting vacuum. Brake stays firm, connect vacuum to booster, brake pedal goes to floor
Noted engine rpm rise when pressing brake pedal to floor, then returns to idle

Eeplaced MC, bench bled new MC, bled brakes again. Same problem, brakes goes to floor when car is turned on.

checked vacuum levels, at temp 17 to 18. I think this is ok.

note that I recently replumb vacuum line to the headlights to get them lifting and returning, lights work great. Could I somehow of effected the brakes?

help, I'm out of ideas

 

Welcome to the forums. The way you describe the booster issue, it's sound as if the brake pedal is not returning to the off position. I believe the only return spring on the brake pedal is inside of the booster and also the master cylinder. It could be the spring or the vacuum valve has issues in the booster is bad. Or there could be an issue with the brake pedal not returning all the way to the release off position. With the car off take your hand and check the Brake pedal, it should return all the way toward the driver and rest against a rubber bumper. And when it's all the way back you should be able to move the pedal toward the apply position and feel a small amount of slack between the brake pedal rod and the booster. By slightly moving the pedal back and forth you should feel slight slack. It almost sounds as if you are applying the bakes when you are not.

 

Thomas has some good suggestions. I will add that I have delt with this before,and it was air the system. It took a lot of bleeding. Good luck!

 

There is always the other option is Change over to manual bakes and do away with the booster. That would mean changing the master cylinder to a 1in bore instead of the 1 1/8in used on power brakes.

 

I have considered going to a Hydroboost system on the 77 that doesen't use vacuum. Whenever it sets for any amount of time and then it's started it probably takes 30 sec. for the vacuum to build, between brakes and the light system (and HVAC system) it requires a fair amount of vaccum. Many options to choose from.

 

Thomas

thanks for the suggestions. I confirm that the pedal does in fact return to the upright position once release. I also do feel the slight play when pressing the brake prior to its engaging the push rod.

I tested the booster as follows. Check valve functional. Booster holds vacuum by testing with the booster connected to a hand pump. With car running, I clamped the vacuum line and pulled the booster check valve out and heard the booster decompress.

I'd rather not bypass the booster and go to manual brakes.

is there another vacuum source in the brake system that perhaps I accidently disabled.

again with the car off and or the power booster disabled, i have pedal that feels normal. Once the car is running, i can push the pedal straight to the floor, lighting the BRAKE indicator.

 

Harley
when you say alot of bleeding, I bled the MC nearly empty (of course not empty) at least 3 times on both back wheels. Am I being impatient?

 

Harley
when you say alot of bleeding, I bled the MC nearly empty (of course not empty) at least 3 times on both back wheels. Am I being impatient?

 

Did you bleed the front as well?

 

Corvettes Like all the Chevys at that time got there vacuum on a single line from the rear of the carburetor behind the secondaries, running to the brake booster, with the vacuum check valve at the booster connection. Very simple. All other vacuum connections are connected to the intake manifold, except PVC Valve and air cleaner. It's normal the brake pedal will go down 1 1/2in to 2ins max travel downward when brakes are applied with the engine running. But the brake light is coming on when braking is not normal. A couple of things to cause the proportion valve to trigger a light has to do with brake fluid imbalance. Master cylinder, brake fluid leakage or trapped air in the lines. Without any obvious leaks and the MC replaced I would say you have trapped air. Might be a good time to flush the system trying to remove all the air. Running a pint of brake fluid through a caliper bleeder is not unheard of. I would bleed it run the engine and pump the brakes over and over maybe 10 to 12 times then bleed them again to see if that improves the condition. I have had proportion valves stick when bleeding brakes and had to clean the valve to free it after it stuck. You must have Air in the system.