With the engine not running, slowly pump the brakes, does the pedal rise and get hard? Now press and hold, does it drop? I shouldn't....then start car, does it drop? It should....

When you replaced the M/C, did you replace the tubes between the M/C and the modulator? Did you re-home the gears?

Hard to believe you lost all that fluid and could not find the source of the leak....

There are two bleeders on the modulator, facing the front of the car.......have some one slowly pump the brakes and hold the pedal, then open one of the bleeders, pedal drops, keep foot on pedal, close bleeder....slowly pump brakes and hold, repeat until no air...same procedure for the other bleeder....

Next do the 4 lines, one at a time, out of the modulator.....just crack one line while someone holds down on the pedal, when pedal reaches floor close line, then pump brakes and hold.....repeat until no air comes out...after the four are done, then bleed the four lines, the same way...rr, lr, rf, and lf....