my breaks are soft but I can not find a leak and have blead all 4 corners. . on 1993 Ford E-150

when backing out of a parking spot the other day, my trucks breaks went all the way to the floor and the truck rolled back about 2 car lengths before the breaks cought rather abruptly.

Asked by for the 1993 Ford E-150
If it has drum brakes in the rear? Make sure they are adjust right. If it still have a spongy pedal? You might have a bad master cylinder?
Does the booster rely on intake vacuum or does it have a vacuum pump?