Bizarre Brake Problem - 1992 100S

[Debu Purohit <purohit@haas.berkeley.edu>]

I'm running into a bizarre brake problem on my 1992 100S.
The brakes groan very loudly when I step on them (sounds like
a large truck is coming through).  However, this
happens only when I'm reversing at slow speeds such as backing out 
of my driveway or into a parking space.  I haven't been able to replicate
the sound when I'm going forward.

The mechanic says that my rear pads and rotors have to be replaced.
That part I knew.  However, he adds that there are lots of
iron filings around the ABS sensor which are causing the
ABS to kick in.  His rationale for the sound occurring only in reverse is that
the metal filings have all lined up in one direction, because most
of the driving occurs in that direction.
Does this make sense?  (I have never felt
the brake pedal pulse when I'm reversing at less than 5 mph.
Although, I have to admit that I've never tried reversing at
higher speeds.)  Anyway, the mechanic recommends changing the rotors
and pads and cleaning out
the sensors to attempt to solve the problem. 
Any thoughts (especially because he wants to charge $500)?

Lacking much knowledge about brakes, I thought the problem I had
was a sticky caliper and/or a warped rotor.  However, it appears that
isn't the case.  Many thanks for your expertise.

Debu
_______________________________
                                                              
Professor Debu Purohit                                         
Haas School of Business                        
University of California                         
Berkeley, CA 94720-1900                       

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