F350 with break problems

Hi, I have a F350 with break problems, and can't figure it out, and since I am on a fixed income, I thought I would try to get some info on here, before spending more money without fixing problem. Problem is that when the truck motor is running, and pressing on break peddle, it will slowly go to the floor, in the pass on my vehicles, this ment that the master cylinder was bad and bleeding back through,... however I have replace the master cylinder with a new one, and still does it, so got yet another one from NAPA, and still the same. I have also replace the ABS cylinder on frame that goes to the back break too, and no help. I have bleed breaks several times, but still no help. When motor is off, peddle seems solid, but when motor running, with vacum buster is working, then pressing on peddle and holding it, it will slowly go down almost to the floor,..... anyone else had this problem, or any ideals that I may not know about.... thanks..... Gene

It sure sounds like a vaccume problem to me, check all of your vaccume lines and make sure they are working right. If they are, i think that you need a new vaccume assist for your brakes. Brakes can be a hard thing to diagnois, so check all vaccume lines and connections.