Brake System Problems. Help Please!!

I recently did some work on the brakes of my '81 GTSi, and now I can't get brake pressure to come up and the Brake Failure light is illuminating. I can't figure out what I could have done wrong! I have checked all the obvious things several times, so I am hoping someone has had a similar problem and it is something not so obvious that I have not thought of.

Before I started work, the brakes were working pretty well. The cable for the parking brake was broken, and there was a little leaking around the adjustment screw on the rear right caliper. I decided to change the pads. The old ones were not very worn, but I did not know how old they were so I replaced them. The rotors were all within spec and smooth, so I did not do anything to them. The pedal was firm and the car stopped straight.

I rebuilt the right rear caliper, repaired the parking brake mechanism, and had to remove the pistons on both front calipers and clean them up because they would not retract for the new pads. They functioned smoothly after I removed the gunk. I also sent the Master Cylinder out for a rebuild. I reassembled everything (correctly I thought) added fluid, and bled all four calipers until no more air was seen. I also bled the master cylinder.

The brake pedal still went all the way down, and the Brake Failure lamp came on when the pedal reached bottom. So I bled all lines again and the same thing happened. And Again. I went thorough the whole system looking for lost motion, adjusting the rear pads again. There were no leaks, pads are all tight, no bulging hoses, but the same problems with the brake pedal still.

If I pump the brake pedal, it rises and is firm and the brake failure lamp goes off. I think that means the new master cylinder is working correctly. The pedal stays hard for as long as I depress it. If I release the brake pedal and wait a few seconds, it goes soft again, travels all the way down, and the Brake Failure lamp comes on again.

I am thinking the problem is somewhere in the system I don't understand. Could an air bubble be trapped at the shuttle valve, or the proportioning valve to the rear brakes? I can't find any way to bleed at those points. Could either of these items be damaged to the point of causing my pedal problems? Since the Brake Failure lamp goes off when I pump the pedal, I think the shuttle valve must be moving freely, but maybe there is something there I don't understand.

As for the proportioning valve, fluid is bleeding freely from the rear screws, so I don't think it could be clogged. I don't know what else could go wrong there.

I have thought about taking it somewhere that can pressure bleed, but since I have put about a gallon of fluid through the system, I can't believe there could be enough air trapped anywhere to cause a problem like this. It seems more like a lost motion problem, but I can't find any.

The only other thing I can think of is maybe I screwed something up when I installed the master cylinder. I know the push rod has an adjustment screw on the end of it, but I did not change the setting of this screw when installing/uninstalling. Could that screw adjustment be causing a problem? Is there a procedure to check it?

Please help if you have any ideas because I don't know what else to try and I need to get the wheels back on my car soon!

Thanks for all your help!

 

Thanks for your suggestions. The pedal does feel like there is still air somewhere in the system. But as I mentioned I have done everything possible to bleed it, so what I wanted to know is if there was something not so obvious that I was missing.

From your posts there does not seem to be anything else I can do but tear the whole system down and start over. I'd hate to find out that it was something simple I just didn't know about.

 

I did replace front seals and cleaned varnished gunk off the pistons and calipers. I can't find any leaks anywhere. The MC was checked again and there is no air present.

Like I said, I was hoping there was something about the system that I didn't know.

 

For some reason, I had to bleed the front and rear brakes at the same time. When I did, I got a full pedal stroke and a lot of bubbles blew out the rear lines and some out the front. After doing this, the pedal was rock hard and the car brakes better than ever.

I have no idea why it was necessary to do it this way, but it worked.