I’ve got some ancient M4’s on my HT, (total overkill for what I use it for nowadays I know) Can’t remember how old they are & I think I got them 2nd hand, but they must be just about the 1st generation 4 pots.
Anyway, theyv’e never had anything done to them apart from replacing pads when required, but they started sticking a bit lately so I did the usual, took the pads out & worked the pistons back & forth a bit, but to no avail.
Took the caliper off (after pushing the pistons out a bit) split it, took the pistons & seals out, cleaned everything with a bit of scotchbrite & brake fluid, stuck it all back together (including the old seals) & hey presto, Work like new!
There again if I’d sent them back to Hope they probably would’ve sent some new ones back.
That’s it really.