93 s4 audi brake warning light comes on, very hard pedal. Then after engine runs a minute or two, pedal very soft, bush down almost to floor two or three times, light goes out, power assist ok. Fluid in master is at corrct level. Bomb replaced last year. Steering assist not affected and stays the same. Servo?
1993 Audi s4 aan
checked fluids: ok
checked for obvious leaks:none
checked to see if steering rack was losing assit: no
problem gets worse when outside temp above 40%
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If the pedal is hard the accumulator (bomb) is bleeding down. If after starting the pedal gets softer, the pump is charging the accumulator. If it's bleeding down, and the accumulator is new, the steering fluid is likely contaminated.
I suggest you have the steering and brake system flushed. These are two parallel circuits driven off the same pump. If the accumulator still bleeds down it will likely require replacement.
Typically a failed accumulator will illuminate the light if you ht the pedal HARD. then you will feel the pulses of the pump assist the pedal.
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Is there any way to check the servo for function? When the accumulator first failed, the pedal went hard and stayed hard. Now, it has this strange situation where if you wait tiill the engine has run a few minutes, you can push the pedal almost to the flor. If you do this several times, the light goes out and all is well. If you forget to do this, the first few stops are pretty scary. hard pedal no assist. Once you have done it though, the light stays out for the remainder of the drive. If you shut down the car for more than a minute or two, you must repeat the procedure.
The sinking to the floor is most likely attributed to a failed brake master cylinder. This is the ONLY cause I have ever experienced or can imagine that would allow the pedal to sink that far.
The loss of assist is the accumulator bleeding down as there is likely contaminants in the internal check valve.
Over 30 years experience, 17 years with Audi, Factory trained