picked up a binnacle lithium 3 w above disc brakes fitted, the other day. not had much use judging by tyre wear, but the drivetrain is v worn and the rotors were covered in oil and no oil inthe system.

now i know these are non servicable cheap enough to buy new etc etc, but i've had them apart and i can't see much wrong other than the piston seal seems quite flat. i have zero experience of bicycle disc brakes (only mc ones), but is that a likely cause of what i assume are/were leaking calipers? it took a fair bit of air pressure to pop the pistons out, but i guess that's nothing to the amunt of pressure the oil is under when braking?

wrt to replacing the seals, if i can't find genuine or pattern replacements what should i be looking for in terms of material (nitrile?) profile? etc etc