5K (Type 44) Brake Question - Hydraulics

[Christopher Ritchie]

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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