Brake Misbehaviour - maybe not ABS?

[Fred Munro]

Hi Peter;

The brake light comes on when the system pressure is low. If it takes 10
minutes of highway driving before the light goes out, the bomb is not
re-charging fast enough. This can be caused by restrictions in the lines, a
weak pump, or a really leaky servo. Even if the bomb is bad, the system
should pressurize rapidly. Ditto the hard pedal after repeated brake
applications - either the bomb is bad and not storing enough charge, or the
system isn't re-pressurizing the bomb fast enough.
I'd suggest the following:
1. Test the bomb: Run the engine until the brake light goes out. Shut the
engine off. Pump the brakes until the pedal gets hard. Count the number of
pumps - you should get at least 10. A new bomb gives 30+.
2. Test the servo: Run the engine until the brake light goes out. Shut off
the engine. Remove the servo return hose at the hydraulic reservoir.
Pentosin should not be coming out of this hose. More than a few drops
indicates a leaking servo which is discharging the bomb back into the
reservoir.
3. Test the bomb check valve: Pressurize the system by running the engine
until the brake light goes out. Check the Pentosin level in the hydraulic
reservoir. Let the car sit overnight and check the Pentosin level again. If
it is higher and the servo is OK, the bomb check valve is leaking, allowing
Pentosin back into the reservoir and discharging the bomb. This is a common
problem in the S-cars.
4. Discharge the pressure in the system by pumping the brakes at least 40
times with the engine off. Remove the banjo bolts in the hydraulic system
and examine closely. Some of these bolts have a fine internal screen in the
oil passages. The interior of the hydraulic hoses can break down with age,
and the rubber particles clog these screens, restricting oil flow, which in
turn prevents the bomb from recharging rapidly. This is a good opportunity
to flush the Pentosin if it looks dirty. Make sure you remove and clean the
screen in the reservoir.

If the bomb & servo are OK and the lines are clear, your pump may be weak.

HTH

Fred Munro
'94 S4

----- Original Message -----
From: "Peter Berrevoets" <pjberr at home.com>
To: "1 Quattro List (E-mail)" <quattro at audifans.com>
Cc: "'David Head'" <v8q at bellsouth.net>; "'Kneale Brownson'"
<knotnook at traverse.com>; "'mike mcclurg'" <rrrrraudi at yahoo.com>; "'Tim
Leonard'" <nard1 at voyager.net>
Sent: Saturday, September 08, 2001 3:09 PM
Subject: Brake Misbehaviour - maybe not ABS?


> Thanks to those who responded earlier,
>
> I finally pulled the fuse on the ABS and have had better, but not problem
> free, brake behaviour.
>
> After checking that the ABS fuse/relay was fully functional, that the
> parking brake lever was fully seated, that brake fluid was topped up, I
> pulled the fuse out.
>
> I haven't checked the sensors on the wheels, or the pressure switch on the
> hydraulic assist thingy, but I had this happen;
> Drove for a while and the brakes worked great - without the ABS - noticed
> that the fluid in the hydraulic reservoir had dropped to below the minimum
> mark - bomb fully charging? - so I topped it up. The brake warning light
> finally went out after about 10 minutes on the highway. A day later, after
> great brakes and no warning light, I was in heavy traffic in town - stop
and
> go - lots of brake use. The pedal started to harden up and the light came
> on. Upon visual inspecttion of the reservoir, it has filled itself right
to
> the top - bomb discharge? For two days now it has stayed like this, and
I'm
> wondering if it is the hydraulic brake assist thingy ot something to do
with
> the master cylinder. The ABS may have a problem relating to dirty
> sensor/teeth, but there is definitely another problem somewhere in the
> system and it's driving me nuts.
>
> The bomb is an almost new unit from a rear ended wreck. The master
cylinder
> and power assist thingy are, to the best of my knowledge, original.
> Hydraulic pump is two year old rebuild, hoses are all tight and dry.
>
> Bentley is just so much greek to me on all this ABS testing with special
> tools I don't have.
>
> Any help would be appreciated - your thoughts?
>
> TIA
>
> Peter
>
> Peter Berrevoets
> 1990 200TQ
> Toronto, Canada
> http://frontpage.home.net/pjberr/
>
>
>
>