5K (Type 44) Brake Question - Hydraulics

[Kneale Brownson]

Are you doing the bomb test with the engine shut off?  If you're getting 
30+ pumps before the pedal gets stiff with the engine shut off, you "pass" 
with flying colors.    Someone posted recently that the alloy brake MC used 
in the Type 44 cars is subject to warping because of engine compartment 
heat and that the warping causes misalignment of the cylinder so that the 
piston can't move smoothly, either on application of the pedal or release 
of the pedal.  The posting dealt with the problem of the brakes not 
releasing at the wheels, but, if correct,  I could see it working the other 
way too.  You say you had the MC rebuilt.  I've been under the impression 
for years and years you can't rebuild an alloy material MC like you could a 
cast iron one because you can't hone the cylinder without distorting it so 
that the seals won't work properly.  You might try this:  Get the engine 
compartment hot and, while experiencing the problems you describe, pour 
some cold water on the MC and see if that gives you instant relief.  If so, 
I'd opt first for a new MC.   They're not all that expensive compared to 
the other parts of the system and they're easily replaced.


At 02:18 PM 07/16/2001 +0000, Christopher Ritchie wrote:

>I am trying to isolate a problem with the hydraulics part of my braking 
>system.  Sometimes, and especially when it is hot, the Autocheck warning 
>goes off for loss of pressure in the braking system.  I know its for loss 
>of pressure because I can feel it in the pedal - it pulses.  Also, I know 
>it is isolated to that reason because I temporarily disconnected the 
>sensor at the servo.  The steering is unaffected.  The pump is rebuilt (by 
>ZF).  The bomb (new, Audi OEM) is 3 years old.  The master cylinder is 
>rebuilt.  The system does not leak.  So, I'm thinking the bomb or the servo.
>
>Do the servo test.  Only a few drops for the return line to the reservoir.
>Do the bomb test.  Seems to be 30+ pumps on the pedal.  But, of course, 
>this is when the car is cold.  The problem occurs on the highway on hot 
>days stuck in traffic.  Can't really do the test there.
>
>Now I'm thinking that is the test.  The autocheck warning sounds for loss 
>of hydraulic pressure even with the engine running.  I can also feel the 
>pedal pulse.  Just sitting in traffic with the engine idling causes the 
>condition.  Turning it off and counting the pedal pumps wouldn't tell me 
>anything different.
>
>So does this mean I need a new bomb?  The new one is only 3 years old.
>Where is the check valve on this system?  How do I know that bomb pressure 
>is not bleeding back into the system?
>
>Bentley tells me that this condition is caused by a bad bomb or a bad 
>servo.  Could there be any other reason?  Does anyone else's 5K experience 
>low hydraulic pressure when the car sits at an idle for a long time?
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