Hey folks, quick brake question for you.

Have a 1977 stock Corvette. The brake pedal was soft and quickly traveling to the floor, and I quickly found a leak in the MC (where it meets the booster). The pedal was soft regardless of whether the engine was running or not.

I replaced the MC (bench bled and feel confident all air was removed) and hooked everything back up. With the car off, I tapped the pedal and it felt great. Sponginess was gone, and it didn't travel to the floor. Exactly how I expected it should feel. Started the engine to give it a good stomp and reset the PV (the rear brakes wouldn't bleed), and the pedal instantly went soft and traveled the floor. Turn engine off, and pedal feels good again. Restart, soft and to the floor. Seems obvious pressure is building up inside the MC with the engine off, but not while running.

Have NOT bled the calipers yet, but wondering if the booster and/or check valve might also be an issue. Thoughts?

Thanks in advance - Steve

 

You have two different systems involved here. The booster is strictly an assist to help a little girl with bare feet lock up all fours. Run by vacuum it has little to do with the other system.

And that is the hydraulic system. Runs on DOT3. It doesn't care if the vacuum booster is working or not. For example: manual brakes.

Boosters seldom fail. Prop Valves seldom fail. But masters, calipers and all the lines can & will fail. You stated you have a firm pedal with the engine off. The hydraulic system is holding pressure. However, the fact that the rears will not bleed indicates a blockage past the Prop Valve.
I suspect there is still air in the system. Lots of air. Its compounded as soon as the booster is activated.

At this point I think only the fronts are engaging. You need to find out why the rears won't bleed. (All four bleeders)
I think I would start with the farthest bleeder and apply a Mity Vac to get things moving. If all else fails, have someone apply some pedal pressure while you loosen the rear / out-going fitting at the Prop V. It should squirt. If not, the Prop V is jammed, shutting off all pressure to the rears.

 

Since the MC had one reservoir low (leading to the rear), my assumption going in was that the prop valve had tripped because of the pressure difference. Hoping I just need to reset it for the rears to bleed. Without getting good pressure on the pedal while the engine is running, how can I reset it?

No doubt there is air in the lines, and I have a Mityvac at the ready. Just gotta get the rear flowing.

 

I don't believe the Prop Valve cares at what level the DOT3 is in the master rez as long as nether one is empty.

The are only two ways to reset the Prop Valve. Complete disassembly. (I don't think you want to go there)
Or, a few quick jabs on the pedal. However, all bleeders must be closed and no leaks whatsoever. If a leak is still present, the Prop V. will sense that and trip itself.
Or in your case, refuse to reset.

Without knowing how old the DOT3 is and how much moisture the fluid accumulated over the yrs, it's very possible the little spring inside the Prop V is being stubborn.
Click on my avatar and look for photos of brake bleeding. You will see a drawing of the inward workings.

If all four bleeders at the rear refuse to give up a drop, then things are really pointing at the Prop V. issue. Or the fitting at the rear of the Prop V.
There is a little tool that might help you with your issue for $12. It's also pictured in my album.

I don't know how comfortable you are about removing the Prop V. for cleaning or replacement. Lots of fittings 40+ yrs old. If a brake line breaks, you could be in for a nightmare towards replacement. If you go that route, start soaking all fittings with PB Blaster.

There is a little tool that locks the piston and may help you with this issue for $12. It's also pictured in my album.

 

Just wanted to thank you for your advice and guidance. I had tried stomping the pedal a few times yesterday (after installing the new MC) with hopes of resetting the PV, but no luck. This morning, however, I gave it a few more stomps and the rear brakes started bleeding immediately. Not sure if it was marinating under pressure overnight or divine intervention, but it was smooth sailing from there. Let me tell ya, there was a TON of air in the driver rear. Passenger rear wasn't too bad, and none at all up front.

Brake warning light is off (finally) and the ole girl stops on a dime now. Took it out for a few miles and no issues at all. Thanks again.