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I've had the same set of Mono Minis for 13 years, they've been on my main MTB for most of that time. I changed the seals about ooh, three years ago or so for the first time.
The other day I found one piston was stuck, so I wet it and gave it a bit of a brush thinking it was gunked up, and it freed up easily and brake feel was improved with two moving pistons. Then a few rides later the lever pumped up after a long descent and didn't pump down again when it cooled. The brake still works fine but somehow the pistons have moved out beyond their normal position and not retracted. Both pistons appear to be moving.
Dirty pistons? Something else? I can't think of the mechanism that would cause this.
Sounds a bit like the pistons have just advanced through the seal a bit further due to pad wear. Maybe they needed to advance a while ago but couldn't if the seals were a bit sticky and a long hot descent loosened them up.
I've had similar with my X2's when the pads are near worn out, my best guess is the seals are running on a rarely used,clean, and smooth part of the piston, and without the friction they self adjust a little too much.
@coatesy they won't be self adjusting too much unless the pad is rubbing. In normal use the piston does not need to move very much and does not slide through the seal. The movement of the piston is allowed by the seal bending and it is the seal bending back into shape that retracts the piston. The piston is only meant to slide through the seal as the pads/rotors wear to take up the slack but sometimes this self adjusting doesn't happen if the seals are sticking to the pistons a bit. In this case you end up with excess lever travel but the brakes still work. Sometimes when the wear has reached a certain point they will unstick as the seal can no longer bend far enough to reach the pads and the pistons will advance quite a lot in one go and the lever will then firm up again.
Edit: I had this the other day after bleeding some brakes and I had pushed the pistons back in. Although the brakes were correctly bled there was still too much lever travel. Tying the lever back overnight kept the pressure on for long enough for the pistons to advance through the seals and take out the slack resultimg in a firm lever.
@Jordan
By 'Seal bending' I think you mean the diaphragm in the lever,not the piston seals.
My take on it is they need stripped and probably new pistons too, which cant hurt to be honest, and you get the impression from their cheapness at a fiver each they are kind of a consumable part being plastic.
I would think its a combination of wear to the pistons, and a build up of dirt inside the brake.
I find greasy deposits in older brakes so i guess any contaminates over the years will have been pushed down into the caliper.
@dyna-ti no I do mean the piston seals. That is how they work to retract the piston.
Well aware of the theories, thanks, unfortunately reality and practice often put the boot into them 🙂
I know how they work generally. But this time the bite point has moved away from the lever much more than it ever has before.
Im going for the sticky piston suddenly slipping thru the seal and going further than usual.
@Jordan
Righto, got you.
I know on cars as the seals are quite big, but I always thought on the bike brake the diaphragm helped to pul it back as its sucked in in a vacuum when you pull the lever. The seals on the brakes stick out such a tiny amount, i didnt think they were that important.
Thanks for the lesson 😀 been servicing these things for years hey ho. Learn something new every day 8)
@Molgrips
I have a nos mono caliper if you are interested
@molgrips am I understanding you correctly? In your first post you said "the lever pumped up after a long descent and didn’t pump down again". To me, "lever pumped up" means the lever has gone from having a lot of movement before braking happens to a little movement. This is usually a symptom of air in the fluid where repeatedly pulling the lever will pump up the brake and reduce lever travel and then the lever will slowly sink back towards the bars and go back to how it was. But, the pistons suddenly advancing through the seals can give the same effect and in this case the lever won't go back to how it was but will stay "pumped up".
In your last post you say "this time the bite point has moved away from the lever". I'm not entirely sure what you mean by that so it may be something different happening.