Proportioning valve: symptoms of being bad

[searcherrr]

Can anyone tell me if they have noticed a popping THUD feeling & sound that comes from their brake pedal when pushed all the way down?

What are the possible symptoms of a bad proportioning valve?

I have replaced every part of my braking system except the proportioning valve and the brake pedal assembly itself. Everything else is new. When I push the pedal down just before it gets to the bottom of the stroke the THUD occurs. What is interesting to me is that if the Master Cylinder is disconnected from the brake booster the THUD does NOT occur.

Would a brake line loose from its mount possibly cause this too? I'm grasping at straws here. The driver rear brake line mount broke off and the line is kinda dangling. I'm gonna fix it of course, though I can't see that this would cause what I'm experiencing. I'm starting to entertain the brake pedal arm bushing as a possible culprit as i have nothing else left to look at.

Could at least 3 people go measure their brake pedal height for me too so I can confirm that the manual is WRONG for FWD ATX NA versions of 2nd gen 3000GT? Instructions for measuring the height are here on 3SI:
http://www.3si.org/forum/showthread.php?t=389038

All ya gotta do is measure from the angled floorboard which is parallel to the brake pedal to the top center of the brake pedal with a ruler or tape measurer. The book says this height to be in spec is supposed to be 7.0 - 7.2" but I believe this is a misprint or is the spec for ONLY TT VR4 models which probably have a bigger brake booster and need more pedal stroke distance.