Problem with XT M785 rear brake

stubs

I bought some XT 785 brakes last month shortened the hoses no problem didnt spill a drop of oil. The front brake is utterly ace so powerful yet with loads of feel I have had to relearn how to brake using one finger or risk headbutting the ground.

The back brake has been a pain when I shortened the hoses I got a good firm feel on the front but the back felt a bit soft so I made up a bleed kit using an Avid threaded adapter attached to a syringe for the bleed cup and gave it a bleed and got some tiny bubbles out. Brake felt firm but a couple of days later it was soft again.

This time I bought the Shimano bleed cup and another bottle of fluid spoke to the guys at Merlin cycles hung the brake vertical and bled it again from start to finish using the Shimano instructions. Same result get a firm bleed next day soft again, tried a method off youtube, tried the old fashioned pump it through with the lever method always the same get it firm and its soft the next day. I tried ziptieng the lever to the bars overnight then zip tied with the bleed cup attached and both times next morning the zip tie had almost fallen off. Even got an old electric toothbrush and run it all over the caliper hose and master cylinder hoping the vibration will shift any stuck bubbles.

Merlin have said send the brake back it might have a leak from the lever seal they have heard of one that went there but I am thinking of having one more go anyone got any ideas I havent tried.

nicklouse

to sum up. the brake is leaking?
or....

sounds like it. send it back.

The Rookie

If you get bubbles each time you have a leak, if no bubbles, you don't!

RevellRider

I've not seen any of the current XT brakes leak from the calliper but I have seen Deore. Get it sent sent back, sounds like it's faulty and will be replaced under warranty

stubs

Its getting one more ride at the weekend then its getting packed up and sent off 1st thing on Monday. Fingers crossed I am not stuck without a brake for too long, couple of weeks Merlin think at the outside.

On the trail its actually not too bad as long as you remember to keep pumping the brake occasionally or you will go flying into a fast bend and the lever will come back to the bar. Not good for keeping my padded shorts stain free

mister p

Check that there is no fluid coming from where the banjo is crimped onto the hose at the calliper end.

stubs

mister p wrote:

Check that there is no fluid coming from where the banjo is crimped onto the hose at the calliper end.

I checked that mister p and I think I might have spotted the problem whilst I was looking for leaks. One of the pistons isnt moving as much as the other its not much but there is a definite lag. Took wheel and pads out pumped pistons out a bit lubed them with some Shimano brake oil and pressed them in and pumped them out a few times. Also dropped in a spare set of pads and cleaned the disc with IPA. Hoping that might have cracked it. Got a ride planned tommorow will bed in new pads and give the brake a good thrashing.

stubs

Think I have cracked it had a 10 mile ride sat night bedded in the new pads and it was better but still not perfect. Sunday morning had a session just pumping pistons out pushing them back with a plastic tyre lever putting a few small drops of Shimano brake fluid on the pistons so it got pushed into seals. After almost an hour both pistons were moving smoothly and evenly. Went for a ride up at Rivvi and no heartstopping Oh shit moments just smooth progressive braking from the rear.

So glad I dont need to send them off for warranty got 3 days planned in Mid Wales in a weeks time.