unbalanced urquattro braking

[ejfluhr@austin.ibm.com (Fluhr)]

Ok, it looks as if my urquattro locks the front wheels first under
hard braking rather than the back, despite the large weight transfer
that unloads the back end (stock, worn suspension).

The front brakes calipers are stock, the rear brake calipers are from
a 1986 4000 quattro.  I know that the brake proportioning valve is
the original, and it shows no signs of leaking brake fluid.  I believe
that the brake M/C is original, as well.  Brake pedal pressure and
response is good, no softness or excessive travel when braking.

I do not believe either of the rear calipers are sticking, although
I have not had the chance to raise the rear wheels and check.  The
rears are doing some braking, since the wheels heat up after a few
hard stops (the fronts get quite hot).

Is this likely to be a bad brake proportioning valve (I have no
experience with the quattro-style valve, is it adjustable)? 
Potentially a bad brake M/C? 

Thanks,
Eric
'85 CGT, '82 urq #87
---
Eric J. Fluhr                                Email:  ejfluhr@austin.ibm.com
630FP Logic/Circuit Design                   Phone:  (512) 838-7589
IBM Server Group                             Austin, TX