Help, Stumped! 5KTQ Brakes

[Bob Blickle]

Hi all,
In a bit of a spot here as I am trying to get the old girl ready for a trip.
First some history...
A few months back the Clutch Pedal started hanging at the bottom. I didn't
fix it right away and got pretty used to lifting it back up with my toe.
At some point my *Pentosin* level got low and pretty sure somehow air got in
somewhere. The Clutch would not stay up at all anymore. I know that sounds
wierd but the Brake Fluid was fine and the *Pentosin* was not. After trying
to bleed the clutch with little success I built a Power Bleeder, changed out
the Clutch Master Cylinder (Slave changed < 1yr ago) and bled the Clutch and
the back half of the Brake system. The front bleeders were stubborn and
technically I didn't have a Brake problem thus far. Checked the Clutch Pedal
it was perfect. Checked the Brake Pedal and it also was great and maybe a
bit firmer. Took it out for some errands and after about 50 miles I start
losing the Brake Pedal. Hmmmm. Took it home expecting to see a broken line
because that's what it felt like but none was apparent and the Brake Fluid
leve was fine. Tried the Brake Pedal after sitting overnight and it was rock
solid. Hmmm. Started the engine and sunnenly no pedal again. Power Bled the
Brake system again (this time completely). Still rock solid with the engine
off and no pedal with the engine running. Went and bought a Brake Master
Cylinder (easiest repair on the car!), primed it using the Power Bleeder,
Hooked up the lines with Brake fluid shooting out, and Power Bled the entire
system (2 full quarts through it) again.  Still rock solid with the engine
off and no pedal with the engine running. Talked with Russ the Bomb guy
earlier in the week and he said he did not think that was it as his car had
normal brakes with a totally breached Bomb.
Ok guys have at it. Where have I gone astray? Thanks in advance!
Bob Blickle