Hey folks,

Gotta dilemma that has stumped a regular ASE shop, the dealership, and
another entirely different ASE shop for over 2 months now.  I have a 1994
GMC 2500 2 wheel drive standard truck, no AC with a 305 in it.  My brake
light keeps coming on, and the pedal gets spongy.
No lines are leaking, I have replaced wheel cylinders twice, it's had 3
different Master Cylinders put on it, and the rear anti-lock has been
replaced.  Not that this matters, but I've had 3 break jobs done on it in
the same 2 month period, pads and shoes.
They, (the mechanic's) have tried power bleeding, and manual bleeding,
thinking that some valve gets in the way if you try to power bleed it, so
then they manually do it, to no avail.  But they DO get air, they think.
After they bleed it, it's good for a day or 2, then reverts back to: Brake
light and spongy pedal.

I am at wits end with this thing, and all I know to do it start switching
parts, what's left. Distribution block, power assist, and calipers.  I think
that's all that's left.
Any insight to what I might try next or what it might be???  This is what I
told each mechanic and they all say the same thing, "It's very very rare
that a distribution block goes bad", and "The power assist wouldn't cause
air to get in the line", and "The calipers would show something wrong if
they were bad".
What the He** is wrong with this thing???

Any and all suggestions are appreciated!!

Thanks guys!

Doug