If the slides are free and the piston isn't sticking, then the caliper is working. If you feel it is sticking. try this. Lift the tire off the ground and secure the vehicle. Spin the tire and hit the brakes. The tire should stop. When you release the brake pedal, it should spin again. If it doesn't. and the caliper is working correctly, something tells me you may have a bad rubber hose to the caliper. They break down inside and will allow fluid to go one direction but it won't return like it should. So, if the caliper is working properly, you replaced the proportioning valve, and checked everything else, that is what I would consider. NOTE:: To check the hose, put the bake on so that the brakes stick. Then, loosen the bleeder to release pressure. If the wheel turns as soon as you release the pressure, the hose is most likely the problem.

Let me know if you have questions.