oh i did it a while ago when i changed break pads, i think it was 4 months ago. i took callipers out, and yeah second piston was stuck, so i pushed it in and out (locked other piston so whole pressure was for second, stuck, piston) so i made em move abit, cleaned the rust, added some grease , and eventually made second pistons move. But still they were not working as the first ones.

but when i break it feels like one side breaks harder then another, then another breaks harder then first one. and so on.