Brake pressures

[Alan cordeiro <cordeiro@avlna.com>]

    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.


    Tom Jervis

Tom,

The hhydraulic pump is TWO SEPARATE PUMPS in one case, driven off
one camshaft. Six pistons feed the power steering through a
"reservoir" which is part of the rear housing, and continually
pump fluid through the power steering system. There is no real 
back-pressure on the system till you turn the wheel to activate
the assist.

The other two pistons feed the brake servo, and once the 
BRAKE ACCUMULATOR reaches 2200 psi, the overpressure
relief valve in the pump opens and "dumps" the excess pressure
back to the input reservoir.

If your fluid is very dirty, the check valve in the brake
servo system might give trouble, and the problem will go away
once you flush the full system.

I'm guessing that the dealer did a flush ( somewhat incomplete )
when he cleaned the filter, but there was still grit all over the
system which has come out again. Try a second ( or first..) 
complete flush of the system. It is really worth a try...

Alan Cordeiro
'86 5kTQ