[s-cars] Brakes seize up when engine is warm

[Peter]

I'll chime in please. First, I will assume that you know for sure that
there is only hydraulic fluid in the hydraulic system and no one has
inadvertently topped it off with something like brake fluid, power steering
fluid, or ATF. The same for the brake reservoir, i.e. that it has only
brake fluid in it and not hydraulic fluid. If the wrong fluid is in the
respective systems they may have lower boiling points  and could be
expanding with heat. If your positive the fluids are correct,  I believe
your symptoms point to the hydraulic brake booster servo.  being
bad/damaged. it is a simple test to discern. Loosen the two thirteen mm
nuts connecting the master to the servo while cold, pull the master toward
you until you can see the end of the actuating rod protruding out of the
servo and take note of its position. Start the car up and see if it  starts
moving forward. If so, then either the servo is bad or something is holding
the brake pedal slightly down. ( I have seen where a wire harness got
between the arm of the pedal and the cruise control actuator and held the
pedal down just enough  that the brakes were gradually set. Very unusual
situation though. )  You can further test it or alternatively test it by
driving the car around until the brakes set;  pull the handbrake on, go out
and loosen the two nuts and pull the mc towards you. Release the hand brake
and if  you are now able to rockmove the car, the servo is bad.  You'll
have to wait again until it cools (it actually is releasing pressure, not
cooling over time unless it has the wrong fluid)
Justinson is more than just a pretty face when it comes to Audi mechanics
but I have to disagree with him here that it is the master, but crow is one
of my favorite meals when talking about Audi.  My. $.01. /pc

On Fri, Apr 13, 2012 at 6:43 PM, Larry <larrycleung at gmail.com> wrote:

> I have a recollection of this occurring on Type 44 turbos on the main
> list. Could be worth a search on the Quattro list. Going purely on memory
> (worth what yer paying for it), it points to a failing brake MC, some
> lister did gstive a plausible explanation on how the engine compartment
> heat caused the problem.
>
> My $0.02,
>
> LL-NY
>
> sent from my iPod
>
>
> On Apr 13, 2012, at 1:17 PM, Thomas Rodriguez <trodri69 at gmail.com> wrote:
>
> > The brakes in my '95 S6 started malfunctioning yesterday and I need
> > some advice on the potential cause.
> >
> > The brakes perform normally when the car is first driven from a cold
> > start.  As the entine warms up the brake pedal engages earlier and
> > earlier until finally there is no play left at all.  At this stage the
> > brake pedal becomes very hard and grabby as though it's getting way
> > too much boost. If I continue to drive I can feel the brakes engaging
> > on their own with progressively more pressure the longer I drive.
> > Eventually the braking pressure becomes so strong that the car cannot
> > be driven without smoking the brakes.  If the car is allowed to sit
> > for a while and cool down the brake function returns to normal.
> >
> > When this happened yesterday the brake system warning lights lit up on
> > the dash board.  The owner's manual instructed me to check the level
> > of the brake fluid and hydraulic fluid.  I did this and found the
> > brake fluid level to be fine but the hydraulic fluid level was way
> > below its "low" mark.  I topped up the hydraulic fluid which took care
> > of the warning lights on the dash board but the brakes still seized up
> > the next time the car was driven long enough to warm up.
> >
> > The car is currently parked until I figure this out.  I would
> > appreciate advice from anyone who has dealt with this sort of thing.
> >
> > Thanks, Tom
> > '95 S6
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