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Re: 5000 brake booster (servo)

     This discussion applies to 5000's etc. with hydraulic power assist.

I've found that failed hydraulic boosters are a fairly common problem. I bet
lots of folks are driving around with marginal boosters. I also "think" that
replacing the pressure accumulator will make a marginal booster look good due
to the higher hydraulic pressure available.  

Anyhow, the telltale booster test is to disconnect the return line from the
booster (the one that goes back to the reservoir). If _any_ hydraulic fluid is
dribbling out of this line when the brake is not applied, the booster is
marginal. The higher the leak rate from the return line, the more "bad" the
booster. Since this is an internal leak, it's easy to understand why folks may
not know they have a problem.

Symptoms of a bad booster are: a bit higher pedal effort (but not too bad) and
lack of pressure reserve. Try pumping the pedal hard 5-10 times while the car
is idling. Can you make your brake warning light come on? Suspect the booster,
especially if the pedal effort seems to remain normal.  Be careful that a bad
accumulator can create similar symptoms. If the booster leaks all the time, the
pressure accumulator may never reach full pressure. Old accumulators that are
OK, but are holding less pressure are "bothered" more by booster leaks.

I got my "new" booster from a junkyard (an Audi with 40K on it). As someone
mentioned, there are 4 nuts that have to be removed from _way under the dash.
You have to remove all of the tricky trim panels, but the actual unbolting
isn't all that hard, just tedious. Be real careful to put penetrating oil on
all of the brake line fittings that connect to the master cylinder (yup, that
comes out with the booster). Bently says that the rod length between the
booster and the master cylinder is very critical. I didn't mess with it.  If
you aren't sure, read the book.

Bently is pretty good in this area,  It explains the drip test I mentioned
above, and has good pictures.

The whole hydraulic power assist idea is kinda neat.  The booster is real small
and light. The hydraulic pump is only marginally bigger than a normal power
steering pump, so all together, there's a pretty good weight/space savings.
BUT, these components just aren't good enough for a production environment.
With the overall attention to durability in an Audi, the hydraulic system is a
real letdown. Good idea, but it wasn't ready.  What are the "newer" Audi's

my 2 cents,