It has started having the brake pedal slowly go to the floor while driving

I have the truck listed above an eco boost. It has started having the brake pedal slowly go to the floor while driving. You can pump it and get the truck stopped but even after pumping the pedal slowly goes to the floor. Diagnostics were checked at the Dealership and was told that the ABS control module was bad. It acted like a bad master cylinder so I changed it first. (Cheapest route) That did not fix it. I could not afford a new ABS Module ($1,300.00) so found a used one with 80,000 miles. Replaced it and still does the same thing. I did not have it reprogrammed yet and the brake lights and ABS lights were on. I decided to put the old module back on but left the newer hydraulic pump and motor on. All lights went out since that module is paired to the truck. We have a code scanner that we use to run diagnostics. All 20 computer systems on this truck have no codes or faults logged. Brakes still do the same thing. I replaced the master cylinder again thinking maybe I got a bad one. (Both are re-manufactured’s) Still does the same thing. Brakes have been bleed twice electronically and still has a slow leak down when you hold the brake. It only leaks down while the truck is running also. Brakes will pump and hold firm with the truck turned on but not running. It is not losing fluid any where. Master cylinder reservoir is full at all times. I’m lost. I could replace the module on the ABS unit with the new/used module that needs programming. I just didn’t feel like the computer component of the ABS unit was bad since it didn’t not log any faults, codes, or lights in the truck and the dealership wants $275.00 to reprogram. When it was tested by the dealership I’m not sure how they came to the conclusion the ABS module was bad because I was not there. It’s my sons truck at college. Help!