MK III Jetta intermittent brakes

[Alchemist]

I just picked up a 94 Jetta, and among other things, it has a strange brake problem.  Sometimes the brakes work OK, but if I hold the brakes on, the pedal gradually sinks to the floor.  Other times the pedal goes to the floor without resistance.  I can bleed the system, and get a lot of air out of both rear cylinders.  Just when the pedal feels like its working, it goes to the floor and I get to start all over again.  There are all new steel lines and there is no sign of leaking anywhere.  The master and both calipers had been replaced recently.  I thought it might be a master, so I replaced it with one from a donor car that had good brakes, without improvement.  My next idea is to bypass the proportioning valve.  Any suggestions or ideas appreciated.

Paul

[jtanguay]

there is a lever on the proportioning valve that has to be actuated and then locked into place when bleeding the brakes.  i have the same problem as you, and i didn't do this, but bled the brakes very well.  i'm going to try this and see how it works.  i still need to replace a stuck wheel cylinder though...