Disc Brakes Lock Up

[FredB]

I recently converted my 60 Apache to disk brakes and late model rear end. After driving 4 to 5 miles the disks are so tight the truck will not move until it sets for a couple of hours. The first time I drove it about 7-8 miles and the front hubs were hot and the truck would not move after I stopped. I bought a new combination valve and changed it out but that did not help. I let it cool and drove it home. When I got home it would hardly move so I cracked open one of the front bleeders and then the wheels would spin. What cuold be causing the pressure to not release? The rotors are used, the calipers and master cylinder a NAPA rebuild. The booster is a new Early Classic (I pulled the vaccum check valve also and didn't help). The front hoses do make a U that is lower than the caliper and the combination (proporting) valve is mounted up and down instead of lengthwise. Could this be my problem?
Thanks
Fred