F250 soft brakes

on a 94 f250 diesel two wheel drive After replacing rear shoes, wheel cly, front brake lines , brake pads. and Bleed the air from the sys. the brake pedal when pushed down just slowly goes to the floor . When the vehicle is off and push on the brakes the pedal is not bad and seems to hold but start the engine and the pedal slowly goes to the floor . I have bled the air out in all kind of ways adjusted rear brack to the drums . plug the master cly off and the only time it will hold a hard pedal is when the front brake line is removed and pluded at the master cly. Then no brake loss. Help!!!

I have an 89 F350/Crew/7.3 Diesel with the exact same problem. Did you ever get it fixed ?

This is NORMAL. If the vehicle stops well (pedal does NOT sink until the vahicle is fully stopped), there is nothing wrong.

The cause is the vacuum pump system. While you drive, the vacuum pump applies a vacuum to the brake booster, but there is NO reservoir of vacuum, and the pump's capacity is not all that great. When you apply the brakes, you lose some vacuum (and the engine is now turning near idle speeds). As you stop, you modulate the pedal very slightly and this uses a little more vacuum. While you are stopped, the vacuum pump keeps pumping, which INCREASES the vacuum pressure on the brake booster, and "sucks" your foot to the floor. If it stops well, don't worry about it.

Newer units use a hydraulic pump for boost, eliminating this concern.

re bleed the brakes , it may take awhile to get it right.
I redid the brakes on my 98 150, there is some sort of issues with the abs block on these trucks.
I went through over a gallon of fluid to get all the air out of my system. I have a very frim peddle now...just like new