bj1k, i hat to say it but i have to agree with the others when i started to read this thread the first thing that came into my mind was the rod length., I suggest that you try it you have nothing to loose and if we are wrong you can always come back and tell us TOLD YOU SO!!!!.

 

I've never seen this before but there is some kind of a spacer inside the master before the short rod goes in making it the same length as the extra long rod that came with the unit, so if you hold the short rod and the spacer side by side with the extra long rod, they are the same length. I didn't even realize it was a spacer in there until I removed the master and it fell out when it tilted. So that seems to eliminate the wrong length rod theory. Back to scratching my head.

 

No

Listen to me carefully.

The booster has two pushrods. One on the MC side and one of the pedal side. Your problem is on the pedal side. That is why I said this.

notice the word pedal

I'm not trying to be a wiseass, but it is a little frustrating on this end when I know what your problem is and you won't listen.

You can also mismatch MC rods, but that won't cause your pedal to rest low and you say you have a spacer in the piston anyway.

Answer me these questions three

1. Where is your brake pedal resting? Is at the upper stop? Climb under there and look.
2. Can the pedal be yanked (with an accompanying *pop*) upwards into the upper stop?
3. Do your brake lights stay on?

 

It's more than that, Roger. There are 2 different rod lengths on the PEDAL side of the booster. And ALL C3s use the LONG PEDAL rod booster. I'm not talking about shallow v/s deep well MC pistons. There have been quite a few people that have had this problem with replacement boosters including myself.

Furthermore you cannot address a length issue on one side of the booster by increasing/decreasing the length on the other side. Doing this means the booster bladder will never fully relax, and you can introduce free play or a preload on the system depending on which way you went.

 

Unfortunately it's not that simple. The travel in the masters cylinder bore is one inch if you measure when bench bleeding. The travel of the brake pedal rod inside the car when pressed by the pedal down to the floor (not from the pedal pad to the floor which is three inches) is also one inch. At the other end where the rod comes out of the booster the rod protrudes 3/8 inch and it also measures 3/8 inch inside the master where the end of the rod seats , which means it should be making full travel when you press the brake pedal. I adapted a 69 style switch to meet where the pedal stops so checking to see if the brake lights are off does not pertain . The booster rod only comes out to where I set the switch and there was a warning tag on the booster saying not to pull back on the pedal rod or damage would result to the booster. I keep coming back to the fact that no amount of bench bleeding will take the bubbles out of the fluid coming through the clear plastic tubes back into the reservoir when bench bleeding. ( And I do mean a lot of bubbles ). It just seems like there is air being pulled around the seals into the master. Remember that when I pressure bled the brakes from the master back , there was absolutely no air coming out of the bleeders but the master won't push it on it's own.

 

Ok, you have done more debugging than I (we) realized then. Mismatching rods is a very common issue with these cars, but it sounds like you have eliminated that as your culprit.

My experience with bench bleeding is that a lot of the time you just can't get those plastic fittings on tight enough (without destroying them) to seal so you always suck a little air on the backstroke which is no biggie. But maybe your piston is totally blown, and you're drawing in air past the rear seal. Though normally if the seal is THAT bad you'll see brake fluid leaking out of the MC from around the piston.

As others have suggested, why don't you try plugging all the MC ports and see if the pedal firms up. If it does, then your problem isn't the MC piston.

 

I agree there should be a better way of sealing those bleed tubes but I guess we just have to live with what they give us, but I have plugged the ports to see if it would hold and it seems to creep down very slow when you hold steady pressure on it. I'm thinking it has to be defective so Sam is sending out a new one. Thanks for your help and I will let you know what I find out.

 

Gotcha, that slow sink is a classic symptom of an MC piston that's letting fluid past it. I'd pull it out ASAP, and make sure it hasn't leaked into your booster. Brake fluid will destroy the bladder in there.

 

Over forty years of working on cars and I still keep learning. This lesson says ( Don't ever assume that the manufacturer is doing their job right ) After having one aftermarket master that was total junk and was making bubbles when bench bleeding and being replaced with a Delco which bench bled out very good, the master still wouldn't pump fluid when installed. What baffled me was that the distance that the booster rod was pushing out and the travel that the master had appeared to be the same, and I was using the long rod which was correct for my car , so it should pump. The problem was that the master only pumps fluid on the last 1/4 inch of the stroke . The first part of the stroke is only pushing the fluid to the end of the master preparing it to be pushed out with force into the lines. ( NOW HERE WAS THE PROBLEM ) The long rod that comes out of the booster that goes into the master was made 1/4 inch too short by the factory that made the aftermarket booster so the master was pushing the fluid and compressing it but stopped just short of pushing it out into the lines, so the result, NO BRAKES. I took a longer plunger rod from a hydraulic clutch and cut it so it was just 1/4 inch longer than the long booster rod that came with the booster, bench bled the master , put the pressure bleeder back on the master ,opened the bleeders until all air was gone and now have brakes so good that will throw you through the windshield. I would like to thank everyone for your suggestions and especially Sam Mooney at Full Throttle Corvettes for his cooperation . He was willing to do whatever it took to find this problem. He is going to contact the manufacturer of the booster to inform them of the rod being too short. I hope this experience can help someone else avoid this aggravation.