Brake pedal goes to the floor slowly on 1994 Ford E-350

I have isolated the front from the rear and determined the front is the cause to the pedal bleeding down. I bleed numerous times. Booster works perfectly, no air leaks. Only rear abs. I also replaced both calipers. To isolate the front, I disconnected the rear brake line from the master cylinder and plugged it. Pedal goes to the floor slowly. I disconnected both lines from master cylinder and plugged. The cylinder locks up, no movement.

Asked by for the 1994 Ford E-350
Use brake line clamps to block off each front caliper at the brake flexible hoses just to eliminate the front calipers/caliper mounts being an issue. It is likely to be a bad brake master cylinder.
Thanks for the tip. I took your advice and clamped the lines. I had no pedal drift with the lines clamped. I have bled the calipers, but is there any special bleeding techniques for a caliper? I removed the left and it bled down. I clamped the left and removed the right, it bled down.
you may need a pressure bleeder to force the air out of the system.

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from your description, either you have air in the lines that is being stubborn, a leak in the front or a bad master cylinder.