Re: Brake system flushing

[zm@mhcnet.att.com]

> What's the accepted procedure for flushing an ABS-equipped braking
> system?  Is there a certain type of pressurized bleeder I should use?
> 

Some ABS systems have an additional nipple at the ABS unit that needs
to be bled as well. But in my 87 5000 CSTQ there is no such nipple and
indeed the Bentley Audi manual makes no mention of it. I bled my brake
system when I re-built the clutch master cylinder (the hydraulic clutch
shares the same brake reservior) some months back.

Just bleed the system as you would bleed a normal brake system. The general
procedure is to bleed the wheels that are farthest from the master cylinder
and work to the closer ones. So, on an Audi 5000, you'd start from the
right-rear, then do the left-rear, right-front and left-front in that order
(since the brake master cylinder is in the left-front part of the engine
compartment, on the driver's side). I don't know what type of Audi you
have, but if the ABS unit has a nipple on it, bleed it in the order of
proximity to the master cylinder.

As long as you haven't taken apart the hydraulic system (to rebuild the
calipers or master cylinder) and introduced air into the system, you should
be able to bleed the system without a pressure bleeder. Just make sure that
the reservior is kept topped up with fresh brake fluid as you bleed. This
will ensure that you will not introduce fresh air bubbles into the system
from the reservoir end. I have found that a "one-person brake bleeder" from
Sears is very helpful. It costs < $10 and attachs at the caliper end.

If your car has a hydraulic clutch, make sure you bleed the clutch line
too, at the clutch slave cylinder.

When I rebuilt the clutch master cylinder on my car, I found that the 
instruction in the Audi service manual to pressure bleed the system to
be very true. When I had taken apart the clutch master cylinder, I realized
that its design makes it difficult, if not impossible, to get rid of the
air by just pumping the clutch pedal. I did not have a pressure bleeder
but improvised one by using my air compressor. I found a plastic cap from
a container that fit on the brake reservoir, drilled a hole in it
and comnnected an air fitting. I pressurized the system to 15 psi and
was able to bleed the system by periodically removing the cap to add more
fluid (time consuming though). So my experience is only with a make-shift
pressure bleeder but I don't see why the clutch and brake systems can't
be bled with old-fashioned pedal pumping. Anyone with more experience,
please cooment.

Oh, based on recommendations of others on this mailing list, I used
Castrol LMA (Low Moisture Absorption) DOT 4 fluid. I think its good.

Hope this helps.

Zafer Mehmood
zm@mhcnet.att.com