If its not one thing its something else. Drove my car this morning and my front brakes locked up. I drove in first back home and bled the brakes and everything was fine. drove it 8 hours later and they locked up again. What the hell? Need new calipers, master cylinder?

 

brake lines the soft ones

 

i remember a member had the same problem. bled it and everything it it comes back eventually. IIRC, it was his caliper piston that seized up and froze. good luck.