randerson

Can't figure this one out. It's just when I'm almost at a dead stop, like the last 5 feet of movement up to the complete stop. Feels like the ABS is coming on and the brakes are pulsing like I'm on snow. Happens every time I come to a stop. No lights coming on, but I notice the red brake light stays on for about 15 + seconds when I first start the car. Brake fluid levels seem fine. Any guesses?

Thanks,
Robert
'93 S4, 123000 miles, bone stock

TRTW

I had to deal with the same troubles with my 98 A4. The advice for me was to pull each sensor from the hub, clean them up and then replace. I had heard that this would clear up the problem. Unfortunately two of my sensors were seized in. I cleaned up the two I could remove but it didn't help. I ended replacing both of the seized sensors. They were roughly $175 a piece with the small mounting sleeves used to hold them into the hubs.

If you can back out your sensors, clean them up with some brake cleaner. Reinstall with a new sleeve. Don't use any anti seize no matter how tempted, it'll disrupt your sensor. And be carefull with them, they're fragile. Hope this helps

Waskly1977

I was having the pulsing and the front driver's side wheel would lock up at low speeds in gravel / dirt.

I just recently replaced a whole bunch of parts and had the axles out. The CV boot had split on that side and there was grease EVERYWHERE. I cleaned up the ABS sensors and the machined part of the outer CV joint. No more pulsing...

The ABS sensor just pulls out (or at least mine did). It's held in with some compression clip. See if you can turn the sensor side to side first. If it moves you can probably work it out by pulling on it and moving it back and forth.

Try to clean out the machined part of the CV joint too. Probably some gunk in there as well.

The brake light staying on for a bit after you get going is probably a sign that the brake pressure accumulator (bomb) is acting up or getting ready to fail. Search the forum for some tips on how to determine it's starting to fail.

Hope that helps

Sloooo4esT

ABS sensors are dirty or misaligned? I have not gotten around to messing with them, and forgot about em last time I did the brakes

popdemonic

I also having this problem.the brake pedal pulsing like ABS activated.Firstly,I guess the bad ABS sensor.I tried cleaning up everything.but it's still pulsing.no any code fault.but i noticed everytimes when first start the red brake warning always on.sometimes you put the hard brake it will comes on as well.
it's clues that the insufficient press. in the system.and i started to find further information.finally,i found out that there's a few s-car got the same symthom as mine.I decided to mail to Fred and he gives me some very good advice...Thank to Fred.

Here's Fred Munro suggestion to my case.....





This is the way the brake boost system works:

The reservoir is connected to the hydraulic pump and feeds oil to the pump.
The pump sends pressurized oil to the bomb. The bomb contains a rubber
diaphragm. On one side of the diaphragm is a charge of nitrogen gas at ~3000
psig pressure. The oil is delivered to the other side of the diaphragm. The
pump pressurizes the oil stored in the bomb to ~3000 psig. There is a
pressure relief valve that diverts oil from the bomb to the reservoir when
full pressure is reached. The bomb also contains a check valve to keep oil
from flowing back through the pump to the reservoir when the engine is
stopped.

The bomb is connected to the brake servo. When you apply the brakes,
pressurized oil flows from the bomb into the servo and applies force to the
master cylinder to assist in applying the brakes. The oil flows out of the
servo to the reservoir through the return line on top of the servo. When the
brakes are not applied, no oil should flow through the servo. If the brake
assist system fails completely, it is impossible to stop these cars with
manual brake application alone. The bomb is intended to provide enough
reserve capacity for ~40 brake applications with the engine stopped as a
safety factor to allow the car to be stopped if the engine stalls.

The pump alone cannot provide enough volume to feed the servo during a brake
application, so some oil is drawn from the supply in the bomb with each
application and recharged into the bomb between applications. If the bomb
pressure or volume is low, it won't supply enough oil and you will feel the
pressure pulses from the piston pump as a vibration in the brake pedal.

Servo Failure

These servos usually leak a little bit and will discharge a bomb if the car
sits for a few days. A badly leaking servo will allow so much oil to flow
through it that the pump cannot charge the bomb to full pressure, and what
little pressure is stored bleeds off to the reservoir as soon as the engine
is stopped. One sign of this is that the (!) light will not go out no matter
how long the car runs. This light is warning of low pressure in the brake
assist system. This is the problem I recently had with my car and could be
the problem with your car. Doing the servo test I outlined in my previous
e-mail will identify a servo with this problem.

Bomb Failure

The bomb can fail in several ways:
1. A bad check valve will allow the bomb to discharge pressure back through
the pump into the reservoir when the engine is stopped. The system will
operate as normal when the engine is running.
2. A bad pressure relief valve can prevent enough pressure from being stored
in the bomb to operate the brake assist system. This could be your problem.
3. Over time the nitrogen charge in the bomb bleeds through the diaphragm
into the oil and the bomb loses capacity. The bomb can only store as much
pressure and volume as there is nitrogen on the other side of the diaphragm,
so eventually (over 10 years or so) the bomb loses enough capacity that it
can't supply enough oil to assist the brake boost. When you apply the
brakes, you feel the pressure pulses from the pump in the pedal because the
pump is trying to carry the full brake assist load without the attenuating
oil flow from the bomb. This could also be your problem.
The test for the bomb is to run the engine until the (!) light goes out.
Stop the engine and start applying the brakes, counting the number of
applications. At some point the brake pedal will get hard and stop higher in
the stroke - from your previous e-mail it sounds like you've felt that
already. This happens when the bomb is discharged. A good bomb will give you
~40 brake applications. A really bad bomb will give you 1 or 2.

With this kind of issue I always test the servo first to rule it out as the
problem. If the servo test good, I replace the bomb. The valves in the bomb
cannot be replaced or repaired.

Note that these cars are very dangerous to drive with a faulty brake assist
system. It is very likely that the brake system would fail in an emergency
stop that draws a lot of oil through the servo for brake assist.

Your description of the "pressure leak" would be the sound of oil flowing
through the servo until the bomb is drained. The (!) light coming on
indicates low pressure in the brake assist system. The pedal vibration is
from the pump pressure pulses when the bomb is drained. The servo should not
do this and this would indicate that your servo is bad. Try doing the servo
test as well. You may have a different servo fault in which it passes too
much oil with each application. The servos are extremely expensive, at least
here in North America.



Here's Fred Munro Test Procedure..


The most likely problems are a bad bomb or a bad brake servo. The servo is
more expensive than the bomb.

The brake master cylinder is bolted to the servo. The servo has two
hydraulic lines attached to it. The line on the bottom is the high pressure
line from the bomb. The line on the top is the low pressure return line to
the reservoir.

You or your mechanic can test the servo. With the engine off make sure the
bomb is discharged by pumping the brakes until the pedal gets hard. Remove
the return (top) line from the servo and screw in a short section of brake
line. Attach a plastic tube to the other end of the brake line and lead the
tube into a bottle. Clamp off the rubber hose part of the return line to
prevent fluid leaking out. Start the engine and DO NOT apply the brakes. If
fluid is running out the hose and into the bottle the servo is bad. No fluid
should be coming out of the line.

If the servo passes the test, it is likely the bomb is bad.