Time for a rebuild mate have had the same issue before with the 4pots pulled them apart cleaned the bores and pistons replaced the seals and orings . Started replacing the fluid in sooner intervals and haven't had an issue Lots of build up due to heat .

I used wax and grease remover to clean the bores and pistons also a good idea to get a set of small cleaning brushes to run through the bleed nipple and brake line ports.