1990 Coupe Quattro - ABS Question

[Mark W. Byrum, Jr.]

Dear Quattro List:
    I am looking at a 1990 Coupe Quattro for possible purchase; 143k
miles, multiple minor problems, but the problem which concerns me is
that the "Antilock Off" light in the instrument panel stays lit, the
"Brake" light just to its left flashes when applying the brakes briskly,
attempting to activate the dash switch to turn the ABS on/off has no
effect, and the brake pedal "feels" hard requiring a good deal of
pressure to slow the car.  Is this a common problem?  Is this a likely
expensive problem?  What are the likely sources of this problem (as
simple as a faulty ABS off/on switch or as complicated as a failed bomb
and ABS system)?  Oh yes, the brake master cylinder looks quite fresh
under the hood.  Your thoughts and inputs will be greatly appreciated.
V.Resp.
Mark W. Byrum, Jr.
'95 Sport 90
'86 4kcsq