brake problems s1900 with hydroboost

i have a 1987 s1900 international with hydroboost. it has disc all the way around. i replaced the master cylinder when i bought the truck. i replaced the font brake pads and had both rotors turned and rebuilt both front calipers. the pass. front got hot and seemed to be sticking. i replaced both hoses on both sides from the steel lines to the calipers. that did not fix it. i then replaced the caliper. it did fine for 2 trips then it got hot again. i then blew out both front lines from master cylinder to the calipers. nothing. i took two short brakes lines and put them above the master cylinder and started the truck and ran it for 10 minutes or so and no fluid came out. i can see when you press then depress the brake pedal the caliper clamps down then releases but i then jack the truck up and it has drag on it. i did check the oil in the hub and its full and cool to the truck so i know it is the brakes. could my hyroboost be weak or what? why is just this one side not releasing fully? there is electric motor below the master cylinder. what is this for?

thanks in advance
shannon