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Re: Audi Brakes
My brake drag has only repeated itself once since my original post and
it was very mild. The hydraulic boost is intermitant when driving for
more than 20 or 30 minutes. I have recieved some confilicting advice
as to what the offending component is and since there seems to be
multiple things happening and I'd rather not spend the money to
replace all of them. The first thing I checked was the servo.
According to Bentley if "hydraulic oil escapes continuously, servo is
defective" and "if only drops of hydraulic oil escape", the servo unit
is ok. Mine loses a few drops every minute or two continuosly. I
assume that this means my servo is on its way out but will a good
servo lose just a few drops of oil when the fitting is first removed
and then hold 2000+psi without leaking a drop? Last night I hooked up
a 5000psi gauge to the servo to see what is happening. The system
charges up to 2200psi quickly with the engine idleing. To test the
accumlator gas pressure, Bentley says to "turn engine off, pump brake
pedal until pressure drops slowly, pressure reading at which gauge
drops rapidly to 0 is gas pressure of accumulator, if pressure is
lower than 435psi then accumulator must be replaced." I think I
understand the reasoning behind this although its not completely
obvious how a lack of compressible gas in the system causes an
incompressible liquid to slowly rather than rapidly lose pressure. My
system pressure remained over 500psi for at least 60 pedal pumps
before the rapid drop to 0 which occured around 100-200psi(accuracy of
my gauge is questionable in that range). According to the book this
is insufficiant gas pressure but whats the big deal? It took so long
to depressurize the system that I would think the intended use of the
gas pressure as emergency boost is still functional. How long does a
new bomb take to depressurize and at what pressure? Does insufficient
gas pressure cause other problems like what I have been experienceing?
I also did the leak down test that is supposed to check the bombs'
check valve. The allowable drop is 70psi in 5 minutes and I measured
200psi in five minutes. This could either be the check valve or the
leaking servo if it is leaking more than normal. My plan is to make a
plug to go in the servo return line port so I can isolate the check
valve. On another note the pressure transducer on my servo points in
towards the engine whereas in the Bentley and other cars I have seen
this points out towards the fender. There is no diference in part
numbers in my book but its not completely trustworthy. Thanks for any
______________________________ Reply Separator _________________________________
Subject: Re: Audi Brakes
Author: Non-HP-rringlie (firstname.lastname@example.org) at HP-ColSprings,mimegw3
Date: 7/2/97 9:43 PM
> Item Subject: Text_1
> Hi Bob,
> About 30 minutes into my drive on a 75 degree
> morning the brakes started to drag and the brake pedal was very hard.
> I didn't try any hard stops and the indicator light never came on. I
> let the car cool down till lunch and drove it again and everything was
> normal. Drove it home and back to work today and everything seemed
> normal. I talked with my parts guy who said its the bomb and it
> should have been replaced anyway since I'm approaching 200k miles.
> The Bentley manual has a whole pressure checking procedure for the
> hydraulic system that I'm considering having done but I thought I'd
> ask you how you made out with your '89 since the symptoms you
> described are very similar to mine. Any help is greatly appreciated.
> Pat Kelly
A quick update on my brake problem. I installed a used "bomb"
yesterday and it DID NOT change a thing with my 89 brake drag. After 5
miles of slow speed neighborhood travels, the car slows down as before,
with greater engine effort required. You can see the boost turbo gauge
actually go up from .4 to .6 when travelling @ 30 mph. The used bomb
reguires a much sharper pedal stab to get the light to blink and after
engine shutdown, 20 pumps seem to be there inreserve.