Brake bleeding advice? XT M785

mickus

Hi all,

I have just installed and hosed a XT M785 rear brake. I followed bleeding instructions from shimano as I couldn't find any specific instructions on installing the brakes from scratch. I assumed it would just be the same process?

The problem I seem to have now is the gap between the pads and rotor is quite large (I've read this is characteristic of this model of brakes) to the point where the movement of the lever seems to be too much. The pads I have are used, so slightly worn, but nothing that should cause that amount of movement.

I bought the lever and caliper second hand, so they have had a good amount of use already.

So is there something I should do differently during the bleeding process? It just feels as though the system needs a bit more oil in it in volume? I'm confident there's no air in the system as when the pads eventually engage the action isn't spongy.

Cheers!

cooldad

Just use them and the pads will adjust.

cooldad

Or stick a rubber band around the bars and levers and leave overnight.

mickus

cooldad wrote:

Just use them and the pads will adjust.

How quickly does this happen? I thought it would be a quick automatic adjustment and several pumps of the lever would do it... but it didn't adjust. Probably me just being impatient, but just a little concerned that the adjustment isn't happening and I might need to clean the pistons or something...

I'll try the elastic band trick tonight!

YeehaaMcgee

how much movement are you talking about?
And, have you realised that there's reach and free-stroke adjustment?

mickus

YeehaaMcgee wrote:

how much movement are you talking about?
And, have you realised that there's reach and free-stroke adjustment?

Considerably more movement than I get with the front brake (also a M785).

I have played with the reach adjustment so the lever doesn't hit the bar, but obviously now the lever is further away from my reach than I would like or is comfortable. The free stroke adjustment doesn't seem to help much at all.

YeehaaMcgee

Hmm. I've not actually changed the free stroke adjustment on mine, so I'm not sure how much effect it has.
Something sounds amiss.
When you bled the brakes, did you use the yellow bleed spacer thing?

Greer_

I doubt you'll need to clean it unless you have a sticky piston.

Could it be due to not enough fluid in the system? When I bleed my brakes I always force a bit more fluid in to stiffen up the lever. If I put in too much fluid, the pistons will move closer together.

mickus

YeehaaMcgee wrote:

When you bled the brakes, did you use the yellow bleed spacer thing?

Yup, yellow block in place. But it was loose in the caliper and had to be held in with the pads pin...

Greer_ wrote:

Could it be due to not enough fluid in the system? When I bleed my brakes I always force a bit more fluid in to stiffen up the lever. If I put in too much fluid, the pistons will move closer together.

This is what it seems like to me. How do you put in more fluid than it takes from the usual bleeding process?

YeehaaMcgee

mickus wrote:

YeehaaMcgee wrote:

When you bled the brakes, did you use the yellow bleed spacer thing?

Yup, yellow block in place. But it was loose in the caliper and had to be held in with the pads pin...

Greer_ wrote:

Could it be due to not enough fluid in the system? When I bleed my brakes I always force a bit more fluid in to stiffen up the lever. If I put in too much fluid, the pistons will move closer together.

This is what it seems like to me. How do you put in more fluid than it takes from the usual bleeding process?

Hmm, the pistons should hold the pad in place.
I'm guessing in that case that it was an unsuccessful bleed.
Before re-bleeding, try popping the wheel and the pads out, and squeezing the lever so that the pistons extend outwards a little, then put it all back together.

mickus

YeehaaMcgee wrote:

Before re-bleeding, try popping the wheel and the pads out, and squeezing the lever so that the pistons extend outwards a little, then put it all back together.

That's done the trick, though I can't help but feel this is a cheat...

As an after thought - would it help to have the free stroke adjustment all the way out when bleeding? My thinking is, that will allow a little more oil to get in to the system?

YeehaaMcgee

No, not really.
The reason pushing the pistons out without the wheel works, is because the brakes are designed automatically adjust as the pads wear out. Imagine it as a kind of ratchet, so to speak.
If you bled them when the pistons were loose against the bleed spacer, then they didn't start off in the right position.
They would have eventually sorted themselves out, but by removing the wheel, you can "trick" them into advancing immediately.

Greer_

Yep, I always end up with too much fluid as I bleed the brakes with the pads and wheel out after one to many mishaps! I just loosen up the bleed port on the reservoir and push the pistons back in.

mister p

You do actually get a better bleed if you screw the freestroke screw out when bleeding.