Problem: 1990 Izuzu pickup 2.3L 2WD. Both front brakes had been remaining engaged (like caliper stick) and not releasing. I had been driving the vehicle for some time not knowing this condition was present, until one day I smelled something burning and noticed the engine was running hotter than usual. The smell was excess oil burning on the outside of the engine (oil pan gasket leak). After lifting the truck, both front tires were extremely hard to turn by hand. I assumed the calipers were bad. I replaced both front calipers, both rotors, new brake pads, both inner and outer wheel bearings and the flexible brake lines to the calipers. Once all this hardware was replaced the front wheels spun freely and easily. I test drove the truck, the brakes seemed to work fine. I lifted the truck and checked again and the wheels were locking again. I then replaced the master cylinder and the proportional valve. Test drove the truck again-same problem. I lifted the truck and tried to turn the wheels-very hard to turn. I noticed experimenting if I open a bleeder valve on the caliper the wheels spin freely and if I open the brake line to the proportional valve the wheels spin freely. Once the brake line is open the wheels spin freely and then once the brakes are engaged the system locks again. I have replaced every component on the front brake system and this problem is still occurring does anyone have any ideas?