The MC (Master Cylinder) was leaking on my '90 Cutlass last year and I
replaced it with one from the junkyard.  This unit has the ABS built
into the MC, it's called a PowerMaster III and was used on GM cars from
'89 thru '91.  GM wanted ~$2400 to R&R this unit and local shop was
asking ~$1300 plus core.  I got it from a junkyard for $350 and it works
fine with one little problem.

The brake lights stay on all the f#$K time and I can't figure out what
is causing this.

I did have a problem like this once before, it turned out to be the
switch mechanism attached to the brake pedal.  Adjusting doesn't help,
but I didn't replace it this time because the problem showed up the day
I changed the PowerMaster III so I discounted this as the culprit.

On the MC itself, there is a switch (of sorts) that slides into/under
the MC and it has two wires on it.  I think this switch (somehow) can
tell when you are out/low of brake fluid and trips the dash "brake" light.

So my question is this, does anyone know of something that involves the
PowerMaster III MC which could turn on the (rear) brake lights all the
time?  I was thinking of something like the inertia switch for the fuel
pump because the "donor" car was in an accident.

Oh yeah, I almost forgot to tell you how I turn the brake lights off
(and on).  I removed the fuse, inserted a wire into each slot and
attached them to a switch which I have located by the console shift
lever.  So when I stop (actually, my daughter is the main driver), you
turn the switch on, then off when you take your foot of the brake.  I
didn't like the idea of a momentary switch because you would have to
have your hand on it the entire time you are braking and I thought that
would be too dangerous.  Right now, the biggest problem is forgetting to
turn the switch "on", but it's been so long that it has become habit.
When my daughter drives my Bronco, she reaches for the (non-existent)
brake switch.

Thanks for any and all replies.