I'm stumped, need some help. Background first. 73 Challenger, stock disk drum power brakes except wilwood proportioning valve when factory unit died. Moved from Abilene TX to San Antonio, once in San Antonio brakes would get super hot in traffic and eventually pedal would bottom out, I assumed crappy old brakes, planned an upgrade and never tried to fix, just stayed away from traffic.

Upgraded front brakes, torched disk off old rotor and turned remainder to make a hub, then used Baer 13 inch rotors and calipers off a sn95 cobra mustang purchased used for cheap. Machined bracket from 6061 aluminum, lines from Cass at Dr. Diff.

Bled crap out of brakes, assumed old fluid crap from boiling, so bled/flushed with fresh dot 4. Pedal is rock hard without booster assist, fluid is clear with no bubbles. Test drive yielded unimpressive results, granny brakes at best. Felt like pedal was bottoming out before enough pressure achieved.

Tested a ton, here's what I have. With assist, apply brakes and you can feel it hit a stop, feels mechanical, even sounds metallic. Applying pressure when on this stop I tried bleeders. Open rear, no effect, open front and pedal falls slowly all the way to the floor even with rear bleeders closed.

Neighbor is ASE Master, he thought master was bad, $25 later no change. Cass at Dr Diff thought plunger adjustment, possibly bad proportioning valve; checked plunger with mic it's good, took prop valve out and replaced with hard line, no change.

It's like linkage is binding, or master is bottoming out, but as soon as you open a front bleeder the pedal falls. So, I'm not out of volume, and I don't have enough pressure to mock anything up. Idea?