Brake pressures

[jervis@lanl.gov (Tom Jervis)]

My problem is with a '91 TQ wagon (20V) with 43,000 mi.  Brake pressure
light comes on in the AM for a period that depends on how long it has been
since last run; up to 30 sec or a minute or more.  This seems to be common
to other models; I have a friend with a V8 that has the same symptom but
for longer.  As I understand the hydraulic system (Mitch Loescher note),
based on the system in my Ur-quattro (sadly since sold) there is a pressure
regulator in the system that drops the pressure for the steering system. so
that the pump/reservoir system is used for both and that most of the fluid
is used at low pressure and at relatively higher volume flow for the
steering while the high pressure side, at low volume flow is used for the
brake boost system.  It is clear to me (though not to my dealer when it was
under warranty)  that there must be a check valve in the system to hold the
pressure in the accumulator and that this is leaking back into the system
(no external leaks).  There are no problems with pressure once the car has
a moment to build it up.

However, and this is the peculiar part, said dealer says he cleaned the
filter screen and that cured the problem.  Indeed it did until a month
after the warranty expired and nobody wants to see me again.  Dealer of
course says whatever was wrong was fixed and this is a new problem!  I have
tried cleaning the screen again to no avail.

I have read the preceeding commentaries on the symptoms of death of the
accumulator.  Am I seeing an incipient demise?  Is the check valve integral
to the accumulator or a separate part?  I admit to not having checked the
parts fiche.  Any advice out there?

P.s. to the person who wants to put separate front and rear diff lock
valves on an ur-quattro.  The easier solution is to get the dash valve from
a 4000Q and run another vacuum line from the rear diff.  The valve fits in
the dash, looks like stock, and allows separate locking, which is nice in
the rain.  The real studpidity of the system is the long noodle that the
vacuum servo pushes on (!) to lock the center diff.   There is a fix for
this also, but that is another story.




Tom Jervis