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Discussion Starter · #1 ·
Hi all,

My rear (left) hand SLX m7100 brake lever has felt off for a while- next to no free stroke and poor modulation which is really annoying being on a hardtail. No amount of bleeding or fiddling would change it so ive been running a shorter lever throw to compensate with the front.

I finally got fed up enough to take a close look so I dissasembled the lever mechnism and the problem became clear because Ive had this issue before. The piston isnt retracting fully meaning the lever is effectively always at least 1/3 of the way into the stroke. When I encountered this before the lever rattled around as there was no preload on the ball and socket interface with the piston. The more expensive levers have a preload which seems to have prevented this, but the effective stroke is still much shorter.

The construction of the cheap levers was pretty simple so I managed to dissasemble it. Nothing I tried fixed the piston so I added a stronger return spring which was enough to get the piston to retract fully but stiffened it up.

Im pretty dissapointed as I only got it July 2020; since then I;ve done around 3000 miles and the brakeset is on their 3rd set of sintered pads. Levers are currently over £40...

Ive tried applying penetrating oil but that hasn't changed anything. Is there any way to get the piston out of the SLX levers? I havent been able to dissemble it beyond removing the lever blade. Or is it just toast unless I deal with the crappy feel?

Cheers
 

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Maybe I've misread? Which 'return spring' have you replaced? 🤔

Once you've got the lever off normally you should be able to pull the lever piston out with your finger tips. If the piston is stuck in the bore then try recessing the caliper pistons to push it up to where it should be.

The return spring is behind the piston. Maybe the seals are swollen or have moved out of their normal position?

Could be the return spring isn't seated properly on the inside end of the lever piston or broken perhaps?

I reckon it's worth trying to get the piston out and see what's going on before you give up.
 

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Discussion Starter · #3 ·
Maybe I've misread? Which 'return spring' have you replaced? 🤔

Once you've got the lever off normally you should be able to pull the lever piston out with your finger tips. If the piston is stuck in the bore then try recessing the caliper pistons to push it up to where it should be.

The return spring is behind the piston. Maybe the seals are swollen or have moved out of their normal position?

Could be the return spring isn't seated properly on the inside end of the lever piston or broken perhaps?

I reckon it's worth trying to get the piston out and see what's going on before you give up.
Thanks, I'll have a go at that. I replaced the internal return spring in the old cheap lever but haven't done anything to the SLX. I'm guessing the piston has swollen, not sure I can do anything if that's the problem though. If only Shimano sold rebuild kits :(
 

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Thanks, I'll have a go at that. I replaced the internal return spring in the old cheap lever but haven't done anything to the SLX. I'm guessing the piston has swollen, not sure I can do anything if that's the problem though. If only Shimano sold rebuild kits :(
Yes, irritating that Shimano don't do rebuild kits. I started using Puloline fluid around 6 months ago and have had zero issues or any seal problems so far.

Suppose there are the knock off piston kits on ebay, not tried them myself but perhaps eventually I'd give them a try but only as a last resort. 🤔 Hope you manage to get it sorted or at the very least find out exactly what's happening.🤞
 

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Just remembered. I did put a piston back in and forgot to put the spring in first (had one of those days). 😄

Bit of a hassle but if you need to you could drain out the fluid from the lever, disconnect the hose and push the piston out with an Allen key, screwdriver, etc.
 

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It is likely not the seals, but the actual piston has swollen. Not certain what causes it, but I've had it happening now on at least three different shimano levers, all of which have seen quite a lot of use and abuse.

Solution is to take the piston out and carefully take out some material with sand paper(400) and/or a fine metal file, from the two thickest parts of the piston, so that the piston can again move freely inside the bore. The piston is plastic, and you only need to trim it maybe couple tenths of a millimeter, so certain care needs to be exercised with this. If you are careful, you can do this without removing the seals even, just make sure to clean it carefully with alcohol afterwards. Removing seals might damage those too, and spares ain't available. Apply some slick kick or similar grease when rebuilding to make it slide in smoothly, without twisting the seals.

1945704

Those are the problem sections, sanding those a bit allows it to move freely again inside the bore...

It is a bit of a hassle, yes, but if you've already taken couple of shimano levers apart, it should not be too big of an exercise and you don't need to buy a new lever.

Edit: Photo added...
 

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Discussion Starter · #7 ·
It is likely not the seals, but the actual piston has swollen. Not certain what causes it, but I've had it happening now on at least three different shimano levers, all of which have seen quite a lot of use and abuse.

Solution is to take the piston out and carefully take out some material with sand paper(400) and/or a fine metal file, from the two thickest parts of the piston, so that the piston can again move freely inside the bore. The piston is plastic, and you only need to trim it maybe couple tenths of a millimeter, so certain care needs to be exercised with this. If you are careful, you can do this without removing the seals even, just make sure to clean it carefully with alcohol afterwards. Removing seals might damage those too, and spares ain't available. Apply some slick kick or similar grease when rebuilding to make it slide in smoothly, without twisting the seals.

View attachment 1945704
Those are the problem sections, sanding those a bit allows it to move freely again inside the bore...

It is a bit of a hassle, yes, but if you've already taken couple of shimano levers apart, it should not be too big of an exercise and you don't need to buy a new lever.

Edit: Photo added...
Ah this might be it! I did think of trying this when it happened before but didnt want to ruin the piston with no spare. Looks like theres metal pistons available on ebay for ~£9 so I might try sanding it first and use that as a fall back. I guess then I can reuse the replacement ones in the future as they shouldnt have the same issue.
 
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