Brakes sticking and I'm at my wits end

Hello. I have a 2000 Mustang GT Conv. 5 speed in my shop right now with all 4 wheels locked up. The customer brought me the car after putting new front and rear pads and rotors on it. He said then the pedal dropped to the floor so he put on new calipers and a new master cylinder. He brought it to me with all 4 wheels smoking after a 6 mile drive to my shop. I put on new front pads, rotors, calipers, and hoses. Still did it. So I checked the system and found that when I released the front master cylinder bleed screw, the front wheels freed up. So I put on a new master cylinder and hydro-boost unit (per customer request on the hydro-boost unit???) It is still doing it. But now all 4 wheels are free until I start it up and then they lock up, even if I start it without touching the brake pedal. I removed the belt to see if the power steering pump was over pressuring the hydro-boost and they still locked up. The only things left are the Abs Block or the prop valve and I don't want to just throw parts at it. PLEASE PLEASE PLEASE SOMEONE HELP ME. If I don't get this car out of my shop by tomorrow afternoon, I have to pay for a rental car until I fix it. The customer is super pissed that we can't figure it out. Thanks for any help in advance.

Thank You for the reply. I have probably flushed 2 gallons of brake fluid through the system and I thought he might have contaminated it (haven't seen any evidence of it but the thought crossed my mind). I will try the master cylinder unbolt thing tomorrow morning. Also, I am not extremely familiar with mustangs, but, I have read about adjustable master cylinder pushrods but the one on this mustang isn't adjustable. It would partially explain the only sticking when the engine is on because of the pressure from the booster, but I'm not sure. My head hurts from thinking about this

My only concern with that is that if there is a mechanical restriction in the ABS block, unplugging it might not make any difference? And then I will be back at square one, you know?

I have been thinking that too. The lead tech used to be a Ford Dealership Technician and he had been on vacation the past couple days (hunting season) so he should be in tomorrow and I'm hoping he can shed some light on the predicament. I will try unplugging the Block tomorrow morning and see if it has any effect on the brakes. And to answer your question, I haven't tried it with just the key on and engine off...