Shimano SLX M7000 Rear Master Cylinder/Lever Piston Issues

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Shimano SLX M7000 Rear Master Cylinder/Lever Piston Issues
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flag EzWazuzi
Posted: Jan 22, 2021 at 4:42 Quote
Hello. I have a problem here. So I bought a pair of titanium lever pistons including seals online, installed it on my brakes, and it's working great especially at the front. However, in the rear lever, when I slowly press the brakes it will go up until the handlebar and the brakes won't work but when I press the lever like the usual pressing thing(which is quick), it works with no issues. Before I installed the rubber rings in the piston, I pre-stretched it with a long nose to insert it to the piston with ease, do you thing the pre-stretching was the issue? I bleed the rear brake a couple of times and changed the caliper but still got the same issue.

Thanks!
flag pbuser2299
Posted: Jan 23, 2021 at 7:55 Quote
EzWazuzi wrote:
Hello. I have a problem here. So I bought a pair of titanium lever pistons including seals online, installed it on my brakes, and it's working great especially at the front. However, in the rear lever, when I slowly press the brakes it will go up until the handlebar and the brakes won't work but when I press the lever like the usual pressing thing(which is quick), it works with no issues. Before I installed the rubber rings in the piston, I pre-stretched it with a long nose to insert it to the piston with ease, do you thing the pre-stretching was the issue? I bleed the rear brake a couple of times and changed the caliper but still got the same issue.

Thanks!

How old's the lever? It's probably scored or roughed up on the inside of the master cylinder, that's the normal cause of any problems.

I didn't know you could get these though, good tip.

Semi related and helpfully I cant remember who makes them, I think someone from eastern Europe, but they do an external seal kit that prevents crap from getting into the lever in the first place, like a diaphragm sort of thing. If that works as promised, that seemed to be the best solution for getting more than 18mths out of a shimano lever.
flag howie44
O+
Posted: Jan 23, 2021 at 10:57 Quote
I would suggest pulling the master cylinder piston back out. Stretching the seal wasn't probably the best idea. I doubt it's the cause of the problem but next time roll it onto the piston with lube and a plastic pick.
Did you install the seals in the correct direction? Those are cup seals and need to be oriented correctly.
If those seals are damaged or the bore of the master cylinder is scored they will not seal correctly.
By the way the slow pull goes to bar ... fast pull works in very common with leaking cup seals. The more pressure behind a cup seal the better is seals (fast pull).
Pull that back apart and check the seals and master cylinder bore.
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