Brake pedal acting odd

[Majorcina]

My friend have a problem with his Urs6 AAN,his brake pedal act's very strange.Sometimes it vibrates all the time(you can feel it when you touch the pedal) and the car lose the brake power.That last for couple of days and then come back after 20 days.He changed 2 times the abs ecu and abs "pump" under the hood.I sopose his problem is brake bomb which is same like on urs4?What you think guys?

[Christophski]

Maybe dodgy abs sensors in the hubs? Not sure but would be my first guess. (I'm not a mechanic)

[Majorcina]

There is no ABS light on,and ABS work properly(not locking wheels on hard brake)

[LHD_Kid]

Sounds like a dodgy brake servo to me...

[Majorcina]

Sounds like a dodgy brake servo to me...

Master cylinder?

[LHD_Kid]

Master cylinder?

No, the servo not the master cylinder.

The feeling on the pedal, is it really as violent as the feeling of the ABS or is it more of a vibration - kind of throbbing?

[Majorcina]

Almost the same like when ABS kick in,but it happens all the time when car in turned ON.He also have some smal play in the stearing wheel i think

[Error404]

Brake servo

[LHD_Kid]

Almost the same like when ABS kick in,but it happens all the time when car in turned ON.He also have some smal play in the stearing wheel i think

My money is on the servo being bad...

[Majorcina]

I am a bit confused since my english isnt perfect,can you post a picture on which part you mean?

[RichLV]

If ABS is kicking in on a level dry road = abs sensors at one/more wheels dead/malfunctioning or the abs ring gear on the outer CV joint is clogged/rusty. Can be easily diagnosed with VAG-COM or VCDS on 1993+ cars.
If the brake pedal vibrates as it is being pressed = dead/soon to be dead brake bomb. Below is a brake bomb. You can't fix it, you can just replace it or ditch the hydro assist altogether as most of us do.


Master cylinders do not have issues on these cars besides leaking, well maybe one of 30 can jam or something, but it's very uncommon.

[diesel des]

Mine started doing this too. Was the servo behind the master cylinder. Convert to vacume.

Sent from my HTC Desire HD A9191 using Tapatalk 2

[Majorcina]

So brake servo,is the bomb? I am not sure on which exact part you think guys

[Rusty]

Brake servo is attached to the master cylinder and is not the bomb. It's almost certainly the servo, from my experience of the same issue.

Try a fluid change or three first, in case there's some gunk causing the issue - it's where the seals wear or get damaged inside the servo, preventing them from sealing properly.

[RichLV]

I had the same symptoms when my bomb died. Servo cylinder was fine.