$#@!#$Brakes!- sticking master cylinder? Help me!!

[J4KQ4me <J4KQ4me@aol.com>]

Got a problem with an 87 5000s w/ ABS. Forever the ABS off light has been on,
but that's not the problem- I think. For some time now, on rare occasion the
brakes, all four seem to stick "ON" This happens only when the car is warm,
and being driven in traffic.  When I say they stick, the brake pedal has a
bout an inch more play than usual, and the pedal is a little harder to
depress.  The assist system is in good working order, and I have just swapped
hydaulic servos to see if the jam-up was in there. I know for a fact that it
is not simply sticking calipers, as I have exhausted that end.  It seems as if
the master is not returning. Any BTDT on this? It is reeeealy driving me nuts.
Not only that it is killing the wearables in the brakes.
 Thanks- Jon.