1995 F250 Brakes - Foot goes to the floor

My friend has a 1995 F250 which has what I think is a weird problem. Has about 250,000km. Recently the brake pedel has been going further
to the floor so he replaced front pads and rear shoes and fluid is fine. Still had to press further to the floor...he replace master cylinder (twice), rear brake pistons, bled the system twice and it has been getting worse. I sat in it and when the truck is not running I can pump up the brakes and they stay up. While the brakes are up I start the truck and my foot goes right to the floor and I can't pump up the brakes. Is there air somewhere? Some dealer say need to pressure bleed the ABS, some say normal old school bleeding is all that is required..help.
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