1980 with power brakes. I changed some brake lines and the brake proportioning valve and then manually bled the brakes per the proper wheel/bleeder valve sequence. Now my power brakes (at least the vacuum booster part) doesn't seem to work correctly.

The brakes were bled manually (had a helper pumping the brake pedal with the bleeder valve connected by hose to a jar of brake fluid) to get all air out of the system.

There is ample vacuum to the booster. What is odd, is when I took the vacuum hose off the booster unit, I still had vacuum in the booster (engine not running). I don't think I've ever seen that before.

Is there something up with the booster? It's relatively new and worked fine before the changes.

Any suggestions appreciated.

Larry

 

Did you change the master cylinder? If so, did you get the correct master cylinder for an '80 Corvette with power brakes?

 

Sounds like the proportioning valve (actually just a distribution block) might not be centered.

 

Larry,
If you removed the check valve that was attached to the vacuum brake booster, (when you heard the vacuum hissing sound) should not have been removed. the sound you heard was is normal and correct, if the system is sealed and tight EVEN with the engine not running. You may have damaged that rubber grommet for the check valve or even the check valve itself. After the engine has been running, the brake booster will hold and store vacuum even after the engine is no longer running UNTIL you press on the brake pedal and can hear it release the vacuum stored. The valve that is in the front of the booster is check valve, only operates in one direction. If this check valve does not allow the booster to store the vacuum charge, you may experience a pedal that is very, very hard. Which means that you are holding vacuum. Usually the pedal will remain high up, and when you press it it does not go far and then to get it to stop it takes both hands on teh steering wheel and feet on the pedal, so you can do leg presses against the pedal to get it to stop...and seems like you are stopping a freight train. I

If you start your engine and while it has run for about one minute, put your head down in the floorbaord on the drivers side and using your hand, press on the brake pedal and listen for a "whoosh" sound. That sound is the vacuum escaping from the booster from the backside...like it is designed to do. If you do not hear anything and the pedal is REALLY hard and stays up high...and does not go to the floor. I would check that check valve and rubber grommet and possible hose going to it from the carburator.

Let me (us) know what you find and post any more info.
"DUB"