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

Shimano hydraulic brakes use mineral oil. Most anything will be fine. Kits and such might cost you a lot considering what you are needing. Just remember the brakes brand new with the new shift lever aren't that much money.

the "flat Seals" are actually square seals, and are designed that way.
the inner face of the seal kinda sticks to the piston.. when you squeeze the lever, the seal BENDS... when you release the lever, the seal makes the piston move back to it's stating point. This is how all disc brake calipers work, including Car and Motorcycle types.
the seal is the "return spring".
any other springs found on the caliper are anti-rattle, anti-squeal. Not all calipers have those rather weak springs.

i rather doubt the caliper leaked oil onto the disc... it was likely from some external source.Chain OVER-lubing or messy lubing of components usually cause the oil contamination of brake discs and pads... cleaning the pads will not remove the oil that has leached into the porous pad material... trust me.. many have tried, and it fails to work.
Just replace the Pads.
the discs can be cleaned.. it involves brake cleaner as a first step, then Isoprpyl Alcohol, and lots of good paper towels... some even use flame heat, but that can warp the calipers..
simply wiping the discs will not be good enough... all those holes in the discs hide oil well.
many just buy new discs when they buy pads.

Semi-metallic pads are the most popular, with decent wear rates, and fairly quiet.... "organic" pads stop the bike quicker, but wear out quicker too... full metallic pads last the longest, but have a nasty habit of squealing like a banshee at times, and are just generally noisy all the time... they also wear out discs faster than the other two varieties do.... .

magical "Brake disc and pad coatings" just make another mess, from what i've seen and heard.

brake quiet is a back-of pad coating that is for car brakes... i have it in my garage, but have never applied it to a bicycle pad, ever.

if you get new discs, CLEAN THEM Before installing/using, ok? they are sometimes hit with a non-rust covering before shipping... Most Disc Makers refrain from the coating now.

there was no oil in them tho, which is why i suspect a leak? the remnants of the oil i can see are a light brown rather than pink, but i guess it could have just discoloured over time. the seals seem fine tho - not cracked or swollen?

or maybe it's just the o ring?

how much oil/dirt buildup was visible on the Caliper?

https://roadbikedatabase.com/bikes/2...3-hybrid-bike/

the bike is fairly new, so WHY someone bled the brakes so soon becomes a good question.

if the brake fluid is "light brown" then SOMEONE changed it out for, possibly, DOT 4 Automotive fluid, or Mixed the shimano oil with another brand of mineral oil..... this is not good... DOT4 fluid can cause damage to the seals by swelling them into soft pieces of junk.

removing the offending DOT4 fluid, cleaning everything with iso-alcohol, then refilling with mineral oil won't correct the damage.
IF it's a min. oil mix situation, cleaning.refilling/bleeding may work.

easiest route? buy a new set of the MT200s... they come pre-bled with shimano red fluid, and are, Usually, very dependable.

and CLEAN THE DISCS a few times, before installing the new parts, ok?

I have those brakes on my Giant MTB, and they work great. Since they're available for about $60 a pair, I would just replace them if/when there's a problem other than pads.

Another fan of the MT200s as they are the factory installed choice on my 2019 Cannondale Quick 1 Disc. Other than wanting matchy matchy with the rest of the components, I have no reason to mess with them, even with more than 5000 miles on them.