Non Returning Brake on Shimano STI Lever

[Bernster]

Hi All, wondering whether someone has come across this before: I've got a relatively new 12 speed mechanical Shimano 105 hydraulic brake lever (ST-R7120), which was working fine until riding in quite heavy (but not exactly biblical) rain recently. I found that the lever wasn't returning correctly after braking, so it wouldn't stay locked on, but it was on enough to rub the pads lightly on the disc. I assumed that the return spring had failed, but checked again a day or so after the ride, and all seems to be working fine again. Could it be water ingress? Is there something else that might have failed or be on the way out? Is it worth dumping a load of WD40 into it to see if it displaces any water, or would I be better talking to the retailer about a repair under warranty (this is a bit of a headache, as I'm not great with hydraulics, and they're not local)? Any advice or thoughts gratefully welcomed - thanks 

[FifeingEejit]

The return AFAIK is done by hydraulics alone, whether air pressure has an impact on the functioning I've never noticed.
water doesn't compress so that or some crud could have been preventing the pads returning fully?

Sent from my IV2201 using Tapatalk

[Sergeant Pluck]

I wouldn't recommend squirting wd40 in. It won't help and you will inevitably get WD40 where you don't want it.

I'd start with a bleed and a full clean of the caliper and pistons. Whatever the issue is, those will need to be done, and it might even be sufficient to resolve the problem. It's vital to avoid a piston coming out too far so if you have any uncertainties about that, just go to a bike shop. Brakes are too important to take chances with. If the parts are under a Shimano warranty, I would think any bike shop could manage that.

Is the brake line routed internally or externally? If the latter, then it's not too difficult to remove caliper, hose and lever and keep them all as a connected unit to send back if you choose to go down that route. 

[Lightning Phil]

Check to see if there is crud or any corrosion on the side of the hydraulic pistons.  It can prevent the pistons retracting the little they do after releasing the brakes.  Some cotton used in a flossing manner can clear it up and restore normal function.

[Bernster]

Thanks all, didn't occur to me that it might be an issue at the caliper end, rather than the lever, but that makes a lot of sense. The ride certainly involved a lot of road crud from some fairly agricultural roads - I'll inspect and clean around the pistons / caliper as a next step, and report back. Cheers again