Proportioning Valve Keeps Tripping?

[RichardJ]

He stated that he is using the speed bleeders. They come with a thread coating, although this coating could have deteriorated. A common problem with vacuum bleeding is sucking air past the bleeder threads unless the threads are wrapped.
He also said he has tried a pressure bleater.

I could see a problem with manual bleeding, if debris is holding the Speed Bleeder Valve open and the valve is not closed, each time the pedal is withdrawn. This wouldn't affect vacuum or pressure bleeding.

My only conclusion from all of the above postings is that the Primary Section of the MC has failed.