I have a 78 SuperCab F250 pickup. A couple of years ago I took it to a shop across the street from my work to have brake pads put on it. It was still fine, but didn't want the pads to get low. I always do my own maintenance, but I was building my house at the time and didn't have a garage so it was worth the money to have someone do it. They've never been the same since. After the 1st time, something gave way in the rear and I had to take it back. They said it was a hardware failure and fixed it. About a week later I started losing pedal and took it back and they fixed them again. About halfway home I thought the truck was on fire as it was bellowing smoke from the back. The guy set the ebrake for some reason after fixing it and I didn't know it was on (automatic truck in park on level ground). so I made him change the back pads again and they did, but within a few days I started losing pedal again. I called the owner of the garage to let him know I was upset with the service and he said that they noticed the proportion valve leaking the last time the truck was in there. I asked why they didn't fix it and he said because I didn't ask him to....
My problem is I can't keep a good pedal. I replaced the proportion valve a year ago and the Master Cylinder twice now (1st was a reman so I didn't trust it). I'm not losing fluid either, so I can't for the life of me figure out what's going on. I don't know if they did something to it, if the new proportion valve is doing something bad or if there is some other issue now or what... But I've been over this thing a few times and just can't find anything wrong with it. The pedal is easy so I'm not really suspecting the vac. booster. I will say, if I give them a pump then hit they are much stronger. any ideas on what I can be missing?

 

Does the pedal sink when you keep your foot on it?

 

If you have a booster, listen for a hissing sound when the brakes are applied. It could indicate a diaphram failure.

 

You referenced brake pads and then rear brakes? Does the truck have a disc brake conversion? If its brake shoe issues in the drum it sounds like they were not properly installed short/long leading edge and probably under adjusted. I would pull the axles and pull the drums and start over.