Ford Repair: F250 Brakes, front brake caliper, rear drum brakes

Question
I have been working on the brakes on my 1993 F250 7.3 L Diesel.  The brake pedal goes slowly to the floor, only with the engine running. After a while both the red "Brake" light and the yellow "ABS" warning lights come on.  I have replaced the right front brake caliper and both rear wheel cylinders, pads and shoes, as well as the vacuum pump and brake booster, and I am on my second new (not rebuilt) master cylinder.  I have bled the brake system (with an assistant)so many times that the fluid comes out looking like new.  What am I missing?

Answer
Seems like you have most of it covered. Does this truck have 4-wheel ABS or rear wheel ABS?

I am pretty sure with rear drum brakes it will be RABS.
If it is RABS, you could have a problem in the RABS valve. One of the symptoms of those was exactly what you describe. You can bleed the brakes forever and the pedal will still go down.

Has the ABS light come on? You can check it for codes which might point us in the right direction.