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

[Tom Green]

You have received several replies from knowledgeable folks on this  
list, Tom,
and I value their opinions on things s-car.  It doesn't fit with my  
experiences,
but they were not with the UrS6 or equivalent brake system.

The experience that most closely compares to your issue happened on a  
Rover
with vacuum boost and your description mirrors that experience  
exactly.  The
dealer replaced the booster they diagnosed as leaking (under warranty)  
which
solved the problem.

The master cylinder failed on my MG and resulted in no brakes unless  
there was
a frantic pumping of the pedal and use of the cable operated emergency  
brake.  I
see this as the classic master cylinder failure because this is a  
failure of the seals
which are the only working parts of the cylinder.  This vehicle did  
not have boosted
brakes, but I don't see where a failure in the master cylinder would  
produce pressure
to apply the brakes.  There is fluid expansion with heat, but it would  
take a lot of heat
to create the pressure required to lock up the brakes as you describe.

I have had a rusty emergency brake cable fail to release and keep  
sufficient pressure
on the brake pads to heat up the caliper sufficiently to lock up a  
rear brake.  The front
brakes will appear to be more sensitive when some of the pressure is  
not required for
the rear brakes, but not the way you describe your braking problem.  
( 5000tq)

Your description of the grabby (sensitive) nature of the brakes points  
more to a leaking
brake booster that just requires a little time (rather than heat) to  
pressurize the master
cylinder enough to start applying the brakes.

You have asked for advice, and I don't have much other than suggest  
expert help.  The
only diagnostic path for a DIY job would be to remove the hydraulic  
boost pressure from
the servo to see if the master cylinder applied brakes by itself.   
There is not a practical
method to do this and it is not safe to drive without brake assist.   
So, you could change
one or the other and see if that fixes the problem.  The master  
cylinder is the obvious
choice, both for cost and labor required, and the previous diagnosis  
is probably correct.
: )

You should also investigate where the hydraulic fluid escaped from the  
system;  Even
though not the cause of this issue, it will result in many other  
problems.

Tom '95 S6
          '95.5 S6 avant
Knoxville, TN

On Friday April 13, 2012, at 2:17 PM, Steve Mills  
<s.b.mills at gmail.com> wrote:
>
> Classic master cylinder failure- the brakes will eventually start to
> self-apply as the car warms up. Makes for very challenging driving in
> traffic after a while.
>
> Steve
>
> On Fri, 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