84 4KSQ - Need advice on brake problem

[Adam Brodeur]

Hello everyone,

I don't often send in posts, but I and my mechanic are both stupped.  Here
goes the problem: Car sat for about 2 years...finally decided to resurrect
the old girl....brakes were in dire need of repair..one of the brake lines
had ruptured.  Replaced all rotors and pads, 2 calipers due to frozen
pistons and/or bad hardware and made new brake lines where necessary.  Bled
out brakes and all seemed to be fine...after some driving, brakes needed to
be pumped to stop...figured master cylinder went bad (no fluid loss).
Replaced master cylinder, bled cylinder and brakes again...worked fine for a
while....then, had to put the brakes to get a high hard pedal....bled brakes
yet again...air in the line and frothy fluid....Done this 2 times and still
poor brakes...no signs anywhere of leakage or fluid loss.  Mechanic says
there is what he believes there is a proportioning valve under the master
cylinder.  Parking brake works fine as do the brakes after a few pumps.
PLEASE HELP!!

Anyways, anyone have any ideas on what is wrong and/or have the same problem
and was able to fix it.  Thanks in advance for any and all responses.

Thanks,
Adam Brodeur