I have replaced mastercylinder, power booster, rear antilock brake valve bleed repeatedly stiil no pedal will feel like its there when truck is off after bleeding feels good turn on truck pedal goes away im at a loss . Bought truck very used i found out former owner hauled a way over sized trailor behind it dont know if it means anything our not but thought id put it out there any ideas ?

 

Sounds like you still have air in the lines or calipers start at the farthest point from the master cyl and work your way closer keeping the master full of course if that dosent do it I would try a power bleeder

 

Did you bleed ALL 4 brakes?
I will move this to the 1st generation Dakota sub-forum.

 

that's because with the truck off you are not using the power booster when you start the truck it builds vacuum in the booster which makes breaking much easier. Is the pedal traveling all the way to the floor if so yes you have air in the lines or a leak. if it takes a lot of force to stop the truck then the booster maybe buggered

 

where abouts are you located?