Here in Southern Ontario, Canada, we can have high humidity for long periods throughout the year. Maintenance schedules recommend replacing the brake fluid every 4 years, due to brake fluid absorbing water and rusting inner brake lines and parts, and the change it causes to the boiling point of brake fluid.
My point being that your caliper pistons may have slightly rusted in the caliper bores and this is causing the sticking. Over the 45 years or so that I've been working on cars, I've run across this condition many times. The piston moves to apply the brakes, but doesn't completely release when the brake pedal is let off, because of the rust ring in the bore (usually at the bottom of the bore) and on the piston won't allow the piston to relax back into the bore.
If the problem is not too severe, with a good cleaning and complete reflush of the brake system, the calipers may still be good to use. Remove the caliper from the wheel, and have someone SLOWLY pump the brake pedal until the piston pops from the caliper. Clean the piston and the caliper bore with 0000 steel wool. If any deep pits are found in the piston, then replace the caliper.
If it doesn't have any deep pits, flush the bore thoroughly with clean brake fluid, lube the piston with brake fluid and push the piston back into the bore of the caliper. This assumes that the rubber dust seal and the piston seal have been cleaned and have no tears, otherwise they need replacing. Stretch the dust seal over the diameter of the piston, and when the piston is properly aligned with the bore, and its backside pressed up against the inner seal, push the piston completely into the caliper, being sure that the dust cover seal properly seats in its piston slot.
At this point, I usually remount the caliper and bleed it, until moving on to the next one , and so on. Once all calipers and wheel cylinders have been bled, I usually go back over all of them again and rebleed them in sequence. This has worked well for me over the years, and only ocassionally have I come across a piston that was too far gone to use again, necessitating a new caliper. Good Luck, should you choose to go this route.