I’ve used Shimano brakes for years and don’t think I’ve had this issue before. It’s a deore lever and 4 pot caliper. Only had the bike 7 months. Brake works fine, no issues with it but noticed the lever wasn’t springing back at the usual rate. Gave it a bleed, still the same. Is this likely to be an issue with the spring in the lever or fluid/air/pressure etc?

Bike isn’t getting much use as I’m mostly on the hardtail. It lives in a cold cellar. Not sure if this is a factor.

Cheers

The piston seals are either dirty (unsurprising this time of year) or knackered.

Pop out the pads, squeeze the lever a bit, not too much so the pistons pop out. Clean the pistons with something like WD40, or isopropyl alcohol. WD40 can damage the seals, but a quick squirt wont hurt just for cleaning purposes.

Then cycle them in and out a bit and then put it back together.

It’s pretty common on Deore at least. Both of mine were replaced under warranty & I know it’s only a matter of time until they do the same again (not out of warranty).

Thanks both. Hopefully they’re dirty rather than knackered. I’ve found deore very reliable in the past so I’m hoping these won’t let me down. Would silicone spray be suitable for the pistons?

Could be the other end too – dirt getting into the lever piston (where the return spring is)past the seal creating friction. Had this on my Shimano Zees.
I’d just use the same fluid (mineral oil)as you use to bleed them to lube them.

Sorry, I should have said. It’s the master cylinder I’ve had problems with. I could take the hose & the lever would still not return.

It seems to be a regular occurrence according to the LBS (Perthshire), so might be the local mud in the piston. It seems crazy to me that you can’t disassemble and clean then.

best for pistons is mineral oil.

I’ve lubed the 4 pistons as suggested, no change. The brake doesn’t drag once the lever is released as the pistons retract at a normal rate. Also put a couple of drops of oil on the push rod at the lever but that made no difference. I suspect it’s a lever issue but I’m not taking it apart!

As it seems to be working ok I think I’ll forget about it.

It’ll be a lever issue and you’ll need to replace it to solve.

Warranty it. Not uncommon on Deores but not something they all do either. Seems to be random.