Hello everyone!

I'm looking for some input on a problem I've been having on & off for quite some time. the problem is that occasionally the brake pedal will go straight to the floor. When it happens, I'll have the system bled and it'll be ok for a few weeks, and at times a few months. So far, I've replaced the ABS pump, the master cylinder twice, calipers, rotors, wheel cylinders, and shoes & drums. I'm at a loss this time. I've heard there is a rear proportioning valve that can cause this to occur. Before I go and spend any more on this vehicle, I thought I'd ask for some help. Any suggestions would be greatly appreciated.

Thanks,
Doug