Brakes Problems

Hello, i have an Astro 94 and i been having trouble withthe breaks on one of the two front wheels. the break will lock itself wihile the van is running and eventually it will unlock itself. I have changed the break pads, the calliper, the hose, the master cylinder, and the problem continues. I was hoping that someone has an idea of what the problem could be. Thanks.

Lots of fun to work in the winter. I think my aproach would be to lock up the brake, (push the brakes), check that the good one relaxed, check the bad one, and if still locked, open a bleeder. If the brake releases, there's pressure still in the line, if it stays locked, the new caliper is bad. For pressure in the line, if the one side relesed, I don't think it would the master, but there are propotioning valves in there the check. Pretty vague, but no-one else posted yet.