My friend is having a problem with her XT m785 brakes. She noticed the rear one felt a bit spongy and would sometimes need to be “pumped up” to work. She brought the bike round and asked me to have a look at it.

It didn’t seem too bad, so as I was pressed for time I just attached the funnel and let the brake dangle vertically. I’ve found this quick bleed quite effective at getting rid of the odd bubble if it is in the lever body after trimming hoses. When I did this a few bubbles came out and lever became firm so I thought great. But then I noticed that as I pumped the lever the fluid was leaking out of the lever body, mainly close to the hole on the handlebar side. It was definitely more than just a drip and if you flicked the lever repeatedly you could actually see it spatter out on to the handlebar.

As the lever was now firm, I took the funnel out and put the bleed screw back in. I then cleaned the lever body with compressed air and wiped it down with alcohol. However I’m still noticing the a bit of fluid building up around the lever body and the lever doesn’t feel so firm now.

Can anyone suggest what might be wrong. There is some evidence of crash damage but nothing too traumatic, at least externally.

[sorry for the long post. I’m in Seattle, so I might be slow to reply because of the time difference]

It’s broken, buy a new one or hope for a warranty.

The hole in the back of the master cylinder is an overflow kind of thing.
If there’s too much fluid in the system it gets exhausted from this hole.
Once the fluid level is correct it’ll stop.

Yeah I was hoping it was some kind of overflow, but unfortunately it seems to be still leaking despite being sat for a couple days and the lever doesn’t feel that sharp anymore.

Does anyone know what would be broken inside? I appreciate spares aren’t available but at least if I could spot a broken seal then I would know it was for the bin.

Thanks.

You can get some spares for shimano, maybe not everything but worth taking to LBS for a look

i’ve had exactly this issue, ended up buying a new lever

Not much to leak in that area other than the diaphragm(seperator unit in Shimano speak), available over here at £3.99 via Madison, part no. 8VC 9806. (Shimano no.Y8VC98060).

Thanks for the details. I’ll contact my friend and check whether she’s happy for me to take it apart more. Is it a common problem that the diaphragm gets torn or is it that they just wear out and stop sealing?