Diff. locks

["J. Smethers" <smethers@fast.net>]

Quattro-philes,

Had our third snow of the year in PA today.  It gave me a good opportunity
to drive the Q hard in the slick stuff after a recent alignment.  Drove as
expected, flawlessly.  I can't get over the control and sure-footedness of
this car.  One problem though, the rear diff. lock light does not come on.
I notice a much louder hissing sound from the center console when I switch
on the rear diff. as compared to the center diff.  I suspect a vacuum line.
What other things may be wrong, and where can I check?
Thanks for any help on this matter, and also for the volumes of assistance
that the great people of this forum have supplied in the past.
Regards, Jeffrey Smethers
smethers@fast.net
'84 318i   '86 4KCSQ