brakes on 1995 Ford E-350

when i first push the brake pedal,it goes down to the floor,i have to pump the pedal 4 to 5 times to get a good pedal,but the firm pedal does not stay,it stillreturns to the floor,i have replaced the master cyl,i also bench blead the master cyl before installing then i blead out the brake lines.If you turn the engine off the brake pedal stays firm and does not go to the floor,what do you think the problem could be.Thank you.

Asked by for the 1995 Ford E-350
It sounds like the brake master cylinder is failing was it a new or re-manufactured master cylinder that was put on the truck? . If there are no leaks a repair shop may disconnect the brake lines from the master cylinder and temporarily put in blanking plugs, bleed the master cylinder and then see if the cylinder can generate and maintain pedal pressure in an effort to prove it is the brake master cylinder that is defective. I have seen cars that had a really bad front wheel bearing as you took turns on a winding road the play in the bearing would allow the brake disc to push bake the brake pad and the next application of the brakes would go straight to the floor. I hate doing it but when the pedal is "bad". pinch off the brake flex hoses one at a time, using a vise grips( that has rubber hose put over the jaws) pinch them off one at a time and press the pedal see if the pedal improves when a particular hose is pinched off, have found several odd brake caliper or caliper mount issues over the years this way.