How much does the vehicle sit without being used? Could be collecting moisture in the caliper.
I had a vehicle that would lock up on one side. Couldn't figure it out. Replaced the caliper and checked the lines and didn't locate a problem. Then I pumped the brakes really hard and went out to look and found one of my flexible lines was bulged. The bulge went down and the brakes worked OK again. There was just enough pressure to hold the one side tight a little longer, and the pedal would also get mushy. New flexibles all around and good to go.
