I would sure check the caliper slide pins to make sure that the caliper has proper movement.
Actually, before doing anything else, I would do a careful brake inspection to the pint of removing the pads and confirming that the caliper slide pins are not sticking and properly lubed. I would also push the pistons in to confirm that they are not sticking. Only after confirming that everything is OK at the wheels would I get involved with the Booster and Master.
