PA Function

["Al Powell" <APOWELL@agcom.tamu.edu>]

The estimable steve.powers@spdg.COM (Steve Powers) said:

> The pressure accumulator is a booster and works somewhat independently
> of the brake system. the pressure accumulator uses engine vacuum to
> work. so when it leaks, it leaks vacuum, not fluid.
> 
> I see no fluid leakage either. If it was fluid leakage, I wouldn't still
> be driving it.

Steve, I can't speak to your specific car, but let me comment re: my 
1990 200.  On this car, the PA is a integral component of the 
hydraulic system.  It is (according to my understanding) used to 
provide power assist (yes, "boost")  to the brakes, and to damp 
pulses from the hydraulic pump by virtue of the elastic properties of 
its pressurized nitrogen reservoir. 

On my car, there is no connection of the vacuum system to the PA at 
all.  It has three hoses: 1) A 2200 psi hi-pressure input direct from 
the hydraulic pump; 2) A lower pressure steering rack connection; 3) 
a low-pressure (gravity feed, really) return line from the hydraulic 
reservoir.  (I have these well memorized, having had it off three 
times...don't ask.)

Typically, when the PA ("bomb") fails, there is no leakage.  There 
was none on either of the failed PA's I have experienced.  It seems 
the failure is leakage of the pressurized nitrogen which is contrined 
inside the PA, causing it to lose its ability to serve as a pressure 
buffer.  The tests in the Bentley are based on its ability to hold 
pressure pumped up to 2200+ psi, and the length of  time before 
leak-down when the eneing is shut off.  Indeed, the PA is tested by 
use of a hydraulic gauge screwed into the brake pressure sensor 
socket on the master brake cylinder.

Of course, the field test is the "pump-down" test to see how many 
times one can press the brake pedal before assist is lost to the 
touch.  Less than 10 means the PA is "todt".

Were you perhaps thinking of a different part?  -OR-  Since I lost 
track of the specific Audi model, am I just barking up the wrong tree 
becuz I have the wrong model????  Been wrong before, will be 
again.....

Al Powell                           Voice:  409/845-2807
107 Reed McDonald Bldg.             Fax:    409/862-1202
College Station, TX 77843           Email:  a-powell1@tamu.edu 
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