HELP... 91' 164 ABS problems

["Keith Christoffers" <christkh@domain.elided>]

    I'm having difficulties with the ABS in my 91' 164L automatic. When I
start
    the car the ABS light is not on. I can rev the engine or let it idle for
as
    long as I want, the ABS light will not come on. When I accelerate it
comes on,
    typically before it shifts (auto) into second gear. I noticed that it
comes on
    more readily when the rate of acceleration is greater. If I accelerate
slowly
    sometimes it wont come on. Up until the light comes on I have pedal
judder and
    I can hear all sorts of ABS activity in the front end. I've checked all
sensors
    and wring short of pulling the rear disks and looking at the sensors
target.
    What could this be? (tell me it's not the hydraulic unit)
    Thanks,
    Keith$

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