The master cylinder has a relief port in it - that relieves brake
pressure when you let up on the brake pedal. It basically opens the
brake circuit(s) to the reservoir. This same port is also how the system
is kept full as pads wear and the caliper pistons move out.

If this port is plugged up, or the pedal isn't returning fully to the
top position, it will cause the symptoms described.

I would check:

1. Is there anything restricting the movement of the pedal? ie - can it
return fully up? (I'm thinking something like the pedal bracket got
bent, or the brake light switch is binding.) It doesn't take much to
cause this condition.

2. Has the entire system had a fluid flush? If not - it should. It's
possible there is some crap floating around in the system and it's
managed to find it's way back to the relief port.

Other than that - we probably need better diagnostics. When the wheels
lock up - do ALL of them lock up? (Temperature of the wheel is usually a
clue..) If it's all - it's doing this at a point common to all wheel
circuits - which narrows it down to the master cylinder (again) or the
ABS control hydraulic unit.

Pretty strange behavior allright.  I see that the Bentley manual does list
this condition under trouble shooting and suggests that it may be caused by
the master cylinder although I don't personally understand how that would
happen.  If it was a seized caliper it would probably only happen to one
wheel unless the car has been sitting unused for a long period of time.  Do
all the wheels get hot or just one??  It seems like you have ruled out the
antiskid controller and the master  cylinder so the only thing left is the
vacuum assist.  You might try removing and plugging the vacuum line the next
time it happens to see if that fixes it.  Just remember that you won't have
the power assist with the vacuum disconnected.
Good Luck and please let us know what the problem turns out to be.

I am experiencing a problem where the brakes lock up after about four
to five miles of travel.  I was told that the problem could be
resolved by replacing the master cylinder.  I did, but the problem
continues.  The only way to get going, is to release the pressure via
the bleeding valves.  This works sometimes for an eighth of a mile or
sometimes up to ten miles.  I removed all the relays from the ABS unit
with no luck.

Is there someone who has experience this problem?  How was it resolved?