Help! Brakes seize after driving for 10 min. or so

[stealthster]

I have a 2001 F150 4X4 with a 5.4 in it. A day after getting my inspection, I drove my vehicle for about 100 miles and it suddenly developed this problem where the brakes would all engage. I let it sit and I could actually feel the brakes release after sitting for 10 or 15 minutes. I could then proceed as normal and was able to get the truck home. I tested the truck again a few times, from overnight sitting to driving for 10 to 15 minutes, the brakes would gradually engage until I could drive no more. I examined each wheel, all were in meltdown so all the brakes are being applied. I researched online and found that bad brake fluid or contaminated brake fluid could cause this problem by boiling over and putting pressure on the brake system. So I bought a gallon of DOT 3 brake fluid and flushed and bled each line starting with the back right (furthest) wheel. After this, I was able to drive for about a half hour with no problem, so I assumed this repaired the problem. Well, the next day I took the truck for work and again, got no 10 min. away and they seized again. Let it sit again, got the truck home again, this time, having the truck in my driveway with the brakes seized, I pulled the brake booster hose and check valve off of the brake booster thinking that this possibly was the problem. It did release pressure from the brake booster, but the brakes were still solid. When this occurs, the brake pedal gets stiffer and stiffer till it barely when be depressed anymore. It sounds like a pressure/valve problem but I don't know how to figure out which part it is. My next guess would be the master cylinder, which I might as well just replace the brake booster as well because I doubt I can remove the rusted bolts without damaging it anyway. I just don't want to buy a new master cylinder and brake booster and have it be something else. Anyway, the details of the F150 are in my sig, it has about 176K on it. Any help is appreciated.

[stealthster]

Ok, well I believe I answered my own question, time will tell. I took off the master cylinder and found brake fluid leaking from the seal. Even after removing the master cylinder, the cylinder was extremely hard to press. I replaced the master cylinder, bled it and tested everything ok after an hour of back road driving.