front discs locking up

Check the "ways" that the pads and caliper slide on. Check both the fixed and floating components. With the pads out, slide the caliper assembly back and forth as to simulate use. If the caliper does not slide freely, there you have it. The caliper is heavy and the "ways" are not well machined, so do not expect them to slide like they were on ball or roller bearings.

If "ways" are nicked or rusted, the pads and caliper could hang. The piston would function normally w/o the pads and rotor. If the "ways" are nicked or rusted, a little work with a medium, followed by a fine flat file will bring them back. A Dremel tool with a sanding roll would work, just be careful not to stay in one spot tool long and leave a divot.