UKC

The rear brake caliper on my eMTB (Deore M6000) seems to have an irredeemably sticky piston.  I've spent a considerable time cleaning both pistons as per this Park Tools video:  youtube.com/watch?v=vQXFFgRButo& but the piston on the 'outboard' side of the caliper stubbornly refuses to retract properly.  Even with the caliper mounted as far 'outboard' as the slotted mounting holes will allow, the pad on that side still rubs.  So, unfortunately it's not really possible to "let the seals be where they want to be" as suggested in that video.

From what I've read, it is theoretically possible to pop a piston out completely and replace it without having to source a new seal.  I'm debating whether it's even worth the effort of trying that, though.  Regardless of whether the piston would (a) not be borked, and (b) go back in, popping the piston out would likely to end up with a lot of oil everywhere 🙁

I do have a pair of M615 calipers kicking around and I think I'd be happy to swap one of those in for the M6000 on the rear.  The problem with this is that I bought them off eBay as part of a used full brake set, and the listing stated clearly that the rear caliper was leaking.  As I only wanted the set for the levers, that didn't worry me.  But I kept the calipers anyway "just in case" - as you do - and now I don't know which was the leaky one ☹️  So I'd be taking a punt on which one to fit, with the possibility of ending up having to swap it all over again.

Does anyone have any bright ideas about how to work out which caliper is the leaky one, without fitting it & riding with it for a while to check?

Of course the other option would be to get a new caliper - and in fact a single M6100 caliper is available for reasonable money (used M6000s seem to be like hen's teeth).  But that seems a little unnecessary given that I have a good caliper to hand - I just don't know which one it is!  (Mind you, having been sat around with no oil in for a few years, it's possible that the seals in both the M615 calipers are dried out and shot anyway.  Silly me...)

In my experience Shimano calipers have always been total ball-ache to service in comparison with other manufacturers. I have a 25ish year old Hope M4 which I've stripped down more than once and is still working as good as new; but I gave up trying to fix the seals on an XT caliper. Unfortunately I'm fairly sure that Shimano have designed their calipers to be intentionally difficult to service to encourage you just to buy new ones.

The only real way to determine which of your spare calipers is the leaky one is to fit them one-by-one to check. If it's a proper leak then you won't need to go for a ride and risk contaminating pads, you can just stick a block in the caliper and squeeze hard a few times, with a tissue you'll be able to see if it's leaking or not (you could even wrap the block in a tissue and then take it out to see if it caught any fluid).

All this requires time and effort and multiple bleeds, so it really depends on how much time and patience you have. Personally if a new caliper is reasonable money I would give in and buy it, and spend more time riding!

I've come to the same conclusion myself.  As an unsuspected bonus, it turned out I could get £5 off an already good price for the M6100 caliper from a certain well-known high street car & bike bits shop, so I have gone down that route.  In the mean time the bike is rideable, if annoying due to the (admittedly fairly slight) rubbing sounds from the rear brake.

One catch (these things are never as simple as you first think) is that the M6100 has the bleed nipple in *just* the wrong place, in that it would end up being more or less obscured by the upper suspension member, whereas on the M6000 it's at the rear of the caliper and thus trivially easy to access.  My solution to this will be to move the perfectly OK M6000 caliper on the fork to the rear wheel, and put the shiny new M6100 caliper on the fork.  I think this actually makes sense anyway, since the front brake is the most important one for stoppage, while the rear is more about finesse and control rather than brute braking force.