that tends to happen if a car/truck sits , exposed to moisture,
calipers and rotors & caliper pistons rust, if not used and yes,
obviously the extent of the potential resistance to the piston in the caliper/pistons rust/corrosion,varies, generally removing the caliper for close inspection, and soaking the caliper in a good spray down with something like a 50%/50% mix of MMO, and WD 40, and then driving the vehicle backwards and forward repeatedly, may and generally will free the caliper pistons and allow them to back off the rotor in operation.
if thats not enough replace the caliper.
calipers and rotors & caliper pistons rust, if not used and yes,
obviously the extent of the potential resistance to the piston in the caliper/pistons rust/corrosion,varies, generally removing the caliper for close inspection, and soaking the caliper in a good spray down with something like a 50%/50% mix of MMO, and WD 40, and then driving the vehicle backwards and forward repeatedly, may and generally will free the caliper pistons and allow them to back off the rotor in operation.
if thats not enough replace the caliper.