I seem to keep losing the back brakes on my K5, but not all the time. I will take off, hit the brakes, have what feels to be all four, accelerate, and then at the next stop I only have front brakes. This seems fairly simple since it's pretty much got to be either a master cylinder or proportioning valve going bad. I have never heard of a bad prop valve though. I've got new rear wheel cylinders in my 14BFF and there is no sign of leakage anywhere, reservoir is full. No ABS, it's an 85', you guys no what I have. So....master cylinder or prop. valve? Or something I am overlooking? As always, thanks.