I have a 79 F250 4x4 6.6l 400ci with spongy brakes. They are firm while the truck is turned off, but as soon as I tried the truck on, they get soft. I've replaced the power brake booster, the master cylinder andtand brake lines from the master cylinder. I've bled the system 3 times without any change. I've even adjusted the push rod on the power brake booster.... No change... Can anybody help?

 

I still believe you have air in the system, doesn't matter how times they were bled if it is not done correctly.Maybe you can give some more detail how you bled them.Does the pedal sink all the way to the floor?
The booster will not cause spongy brakes. Make sure your drum brakes are adjusted properly before you test drive..