The brake push rod is maybe pushing too hard on the servo so its operating slightly all the time. There is a specification for the gap between the rod and the servo with the pedal up.. If its the same garage each time maybe they repeat the same mistakes.
How many seals are in it ..is that large ring a rubber seal too ? Also what did you lube it up with ..red rubber grease ?
From memory, two piston seals and the o-ring. I’d doubt you’d get replacements for the pistons. Just lubed it with brake fluid and put it back together with a new paper gasket. Hasn’t leaked.
