Shimano RS785 hydraulic caliper

[tincaman]

I have been playing with the rear caliper trying to reduce the lever travel, but unfortunately made the onboard piston pop out too far which then started leaking from the piston. The trouble is that I can't get the piston to go back in far enough, I have drained the system down and taken the caliper off, stuck a tyre lever in there but it just won't go back in. The other piston goes in almost flush but this one sits out about 2mm, if you reinstall everything it works ok but now the caliper is off centre so I can't get it not to rub the disc. It's not the pads rubbing the disc, it actually the disc rubbing the caliper itself.
Any ideas how to get that piston back in properly?

 

[goody]

Never done it with cycle clipers but worked on loads of m/cycle ones. My fear is you've displaced the piston seal which is why you can't push the piston all the way back in (suprised it's not leaking). You might have to pump that piston out then split the caliper and see whats going on with the seal. Apparently you can't get seal kits, also apparently the pistons and seals are the same as in a new Deore caliper which is about £18. Is the bike under warranty. (I take it the bleed screw was undone when you were trying to push the piston back in?).

 

[lpretro1]

You will need to dismantle the caliper - pop the piston back in (bit of silicone lube or hydraulic oil - of the correct kind of course- eases it in) and then bleed the brake. You can't affect lever travel by playing around with the pistons - if there is any adjustment possible there may be a reach adjust screw somewhere at lever end - consult the technical docs

 

[lpretro1]

Never done it with cycle clipers but worked on loads of m/cycle ones. My fear is you've displaced the piston seal which is why you can't push the piston all the way back in (suprised it's not leaking). You might have to pump that piston out then split the caliper and see whats going on with the seal. Apparently you can't get seal kits, also apparently the pistons and seals are the same as in a new Deore caliper which is about £18. Is the bike under warranty. (I take it the bleed screw was undone when you were trying to push the piston back in?).

OP said he popped the pistons out - you can't claim that under warranty

 

[tincaman]

I'm going to have to drive the piston out then to reseal the o-ring. I can probably do that by taking out the drive side pad and then pumping the brake. It looks like you don't have to come very far out to go last the seal. I have nothing to lose so I might as well have a go.

 

[lpretro1]

If u don't know what u r doing you'd be best taking it to your LBS to get this sorted tbh.

 

[tincaman]

At the moment it's all assembled, but the piston sticks out about 1 to 2mm too far. Looks like there is a leak too around the piston. It's all simple mechanics, but there are often tricks and shortcuts which is why I asked on here before trying anything more drastic.

 

[lpretro1]

Ok - take something like a ring spanner (nothing sharp as you could damage piston). Apply even pressure across the face of the piston to press it back into the slot. It should press home fairly easily - if it doesn't then it is possible the piston seal is not seated correctly or twisted so don't force it or you'll damage the seal. Then bleed the brake.
If it won't push in then you will have to dismantle the caliper and re-seat the seal and piston then torque caliper bolts up and then bleed. I assume you have the appropriate bleed kit and fluid (Shimano uses mineral oil). Take your time - don't rush it or it will end in tears

 

[Yellow Saddle]

If u don't know what u r doing you'd be best taking it to your LBS to get this sorted tbh.

Why? What's wrong with trying, exploring and even mess it up? At least then he would have learnt something, discovered something, had some fun and know better for next time. I bemoan this attitude of if you don't understand something, you should not go near it.

 

[Kajjal]

Why? What's wrong with trying, exploring and even mess it up? At least then he would have learnt something, discovered something, had some fun and know better for next time. I bemoan this attitude of if you don't understand something, you should not go near it.

Fine to a point but be careful of doing avoidable costly damage.

 

[rusty bearing]

I suppose the big problem with messing up hydraulic stuff like brakes comes when you are flying downhill and summat pops.
By all means have a play, see if you can fix it....... then ask yourself 'do I feel lucky'

 

[goody]

If the pistons almost all the way back in but it's leaking then I'd guess the seal is misplaced. Its probably stretched now so not much to lose in taking it apart and having a look. You might be lucky and get it re-assembled or you might not. Must go cheese on toast has arrived and follow the money is starting.

 

[tincaman]

Thanks I will give all that a go tomorrow

 

[lpretro1]

Why? What's wrong with trying, exploring and even mess it up? At least then he would have learnt something, discovered something, had some fun and know better for next time. I bemoan this attitude of if you don't understand something, you should not go near it.

Brakes are a safety item - what the OP was trying to do in the first place shows he didn't really know what he was doing ( no offence to him I might add) so I have to agree with what 'rustybearing' said

 

[lpretro1]

Thanks I will give all that a go tomorrow

Don't forget to clean everything off with either some isopropyl alcohol (IPA) or some disk brake cleaner before u put the pads in to avoid any possible contamination