the pistons often seize onto the piston seal and when moisture goes past the piston boot, the chromed piston begins to pit. again may require replacing.
the calipers handbrake lever shaft seal tends to degrade, allowing the grease in the handbrake cam mechanism to harden, rust & seize which either causes the caliper to bind or the handbrake ineffective.
either overhaul the caliper urself or buy a new one.