front caliper locking up

Out of all the bikes I have rebuilt I never had a problem like this. After 600 miles of being rebuilt the caliper is locking up.
when cold not run a few day front brake is fine rolls smooth brake works, After a few miles down the road the caliper slowly locks up and lever is just about locked won't move. Master cylinder has been previously rebuilt as was the caliper. My thought it's the master cylinder not releasing pressure ?? any thoughts
gregz
note. It worked fine the first 600 miles just started this weekend
Zeds not happy right now

Have you checked the little hole in bottem of mc reservoir to see if its plugged?

You might have moisture in the caliper . When that happens the heat from friction causes steam and it expands locking the brakes. I always carry a wrench with me when trying a new bike out in case I didn't get all the moisture out. It is worth a try .

Ive seen an over filled master cylinder lock the front brakes up in real hot weather at Daytona once