Locking Brakes-Help

[RPurdy]

Ok guys my work van is a 2000 E350 Powerstroke. Ive had very little problems with it until now. The front brakes are locking up. Sometimes they lock up and stick and wont release at all. Other times they work fine. I'm not sure how to fix it because I don't know whats wrong exactly. Someone said there could be a problem with the master cylinder but I dobt that. Someone else said the rails the brake pads slide on is probably rusty. The van did sit for a while while I was away. I haven't taken them apart to look yet because I don't have time at the moment to take half the van apart. Has anyone had this problem or know whats causing it?
By the way the rear brakes are disc to and they work fine. Its kind of hard to diagnose because they only lock up if you drive it for a few minutes (like 10-15) but once they lock up its impossible to drive because the front brakes are locked so I cant go out and drive it to test it, so I'm in a bit of a pickle