M785 replacement - part or whole.

sandy771

I have a set of XT M785 brakes and the front has been playing up over the last few days. Today it has finally given up and squeezing the lever does nothing. I understand that the seals on these brakes are prone to deteriorating after a year or so (these are about 18 months old) so I suspect that this is the problem.

I see that I can buy a complete lever and caliper for about £65 or a lever (and resevoir) for about £24 on chain reaction.

My thought though is that if the seal has gone at the resevoir end (there does seem to be a little fluid here) that they are likely to go at the caliper end as well in the near future. So my question is would it make sense to change just the lever end of the assembly or should I change the lever, cable and caliper?

My second question is that as i purchased front and rear brakes at the same time should I be looking at changing the rear brake as well before it lets me down when i am out riding?

cooldad

Are you sure that's the problem? First I've heard of seals only lasting a year - I know plenty of people with them, including me, and no one has had problems.

The Rookie

Clearly squeezing the lever doesn't do 'nothing' - perhaps if you tell us exactly what does happen we can help....

sandy771

Problem initially was that brakes were on (more than just binding a bit) waggling caliper with hand caused them to release, so I initially though stuck/sticky piston. But prob returned.

Take wheel and pads out - squeeze lever, no resistance at all, pistons doesn't move at all.

update - just to be clear

If I put the pads in and open them wide with a screwdriver and mount my wheel then brake lever extends to fully off/out position and when I pull it the pistons move and the pads now clamp the disk, but very gently - i.e. no real pressure on disks and I can move the wheel by hand with the lever fully against the grip. The lever then stays against the grip when released.

ricardo_smooth

The fluid is probably shot and needs replacing. The mineral oil breaks down and when it's bled it will be a grey/black colour instead of the bright red and it will have 'bits' in it too.

The Rookie

How did the pistons get stuck on if they weren't moving? The account doesn't stack up.sounds like some basic maintenance is required......the mineral oil doesn't 'break down' either.

ricardo_smooth

The Rookie wrote:

the mineral oil doesn't 'break down' either.

Ok word choice was poor. It does become contaminated though

sandy771

The Rookie wrote:

How did the pistons get stuck on if they weren't moving? The account doesn't stack up.sounds like some basic maintenance is required......the mineral oil doesn't 'break down' either.

I suppose they could have been sticking occasionally rather than stuck - it does sound weird but I have to give the full account or you might miss something.

The account however is what is happening, no point in me making it up if I am asking for help!

New front XT M785's brake set ordered from CR

The Rookie

I'll pay the postage on your old ones so I can fix them up!

sandy771

I'll keep them thanks

Getting a new bike with avids, which I'll prob want to change with those currently on my trance, so if I manage to repair them myself slow time I can keep them for my old bike.