Can You Use Shimano MTB Hydraulic Brake Calipers with Shimano Road Levers?

buckmulligan

I'm in the midst of a mechanical crisis here!

Just travelled back from a weekend away with my road bike that's equipped with Shimano hydraulic discs (ST-RS685 levers and BR-RS785 calipers). Unfortunately the lever got depressed when the front wheel was out, the brake was binding, so I removed the pads and tried to push the pistons back in, only to put a crack down the middle of one of the white ceramic(?) pistons. Mineral oil leaking everywhere.

Just had a look online and sourcing a new BR-RS785 caliper looks to be impossible, out of stock everywhere and I need a rapid solution as I'm departing on Friday for a weekend of racing followed by a mini-cycling holiday in Wales.

What are my options here to get myself back on the road ASAP?

I have a mountain bike fitted with Deore hydraulic brakes BR-M595 calipers, is there any chance I can just switch this over? Or even better, can I take the piston out of that caliper and use it to replace the busted one in my RS-785 caliper? If not, does anyone know of a compatible caliper for the ST-RS685 levers?

I've done mechanical work on Shimano hydraulic systems before and have all the gear, just wondering about compatibility. Any help would be great, thanks!

crossed

Shouldn't be a problem I wouldn't think.
As far as I'm aware the RS785 calipers are the same as the XT MTB calipers.

buckmulligan

Thanks for the reply.

That's what I was hoping, Shimano are pretty good about cross-compatibility with their stuff. I'm just concerned primarily about lever travel and secondarily about heat-dissipation; as you say an M8000 caliper with the finned-pads would be good, if they work.

chrisw12

As above, I've read that the early road calipers and MTb calipers are exactly the same, not sure how true this is though.

If you do get it to work, can you post back on here, as you said, trying to find the road calliper is impossible but there are loads of the mtb calliper.

buckmulligan

Sure, no problem. I'm going to head to a bike shop at lunch to pick up a barb and olive and give it a whirl this evening.

I'm hoping it'll just be a straight swap, they're both post-mount and direct hose connection, so it should fit but whether it works is another matter! Whilst the XT and SLX calipers might look similar, they both have a banjo connection for starters, which would complicate things.

I just want the least-faff solution to get me on the road with some functional brakes for now!

veronese68

BuckMulligan wrote:

I just want the least-faff solution to get me on the road with some functional brakes for now!

I wouldn't get into swapping the piston, just swap the caliper. It should work, maybe not exactly the same ratios but you will have a functional brake I'm sure.

buckmulligan

So this has been a partial success so far.

I switched the Deore caliper over to my road bike, bled the system as per Shimano's "inject fluid in the caliper method", has a nice solid lever feel. Refitted the brake pads and wheel and came to align the caliper and rotor and this is where I hit a bit of a stumbling block; after pumping the brake lever to advance the pads, the pad clearance seems really tight and I'm finding it difficult to get the caliper aligned without any rotor rub.

I must've spent nearly an hour at it last night trying every different method of caliper alignment and just can't quite get it perfect. I've had it so that it's spinning freely but then after a couple of hard braking sessions, it's rubbing again. I've torqued up the caliper mounting bolts tighter than they should be and it's still happening, very frustrating.

One thing that is immediately obvious is that the lever has a lot less travel now than it did before the caliper switch and bleed. Can I just undo the bleed port on the lever, let out a drop of two of hydraulic fluid and will this trade some lever travel for extra pad clearance, or does it not work like that?

Any help gratefully received as I'm desperately counting down the hours til I need to depart now!

trekvet

Don't know, but are there not instructions online? Could be that insufficient resorvior air space is stopping the push-back? Have you tried twisting a screwdriver to check the pads are able to go back? Did you tighten the calipers with brake hard on, at same time compensating for caliper move as the bolt turns? I have my wheel bearings free play only just lost by correct skewer tension, in the hope that any disc rocking will help push pads back, works for me.

How about put 2 slips of card in with pads during set-up, then remove last?

buckmulligan

TrekVet wrote:

Don't know, but are there not instructions online? Could be that insufficient resorvior air space is stopping the push-back? Have you tried twisting a screwdriver to check the pads are able to go back? Did you tighten the calipers with brake hard on, at same time compensating for caliper move as the bolt turns? I have my wheel bearings free play only just lost by correct skewer tension, in the hope that any disc rocking will help push pads back, works for me.

How about put 2 slips of card in with pads during set-up, then remove last?

Thanks for the reply; I've tried the "brake-on-and-tighten-caliper-method" and that's nowhere near accurate enough, the pad clearance is really too tight. The best I've managed is looking down the inside of the caliper and keeping an even pad-spacing on either side of the rotor as I tighten it, but it's still rubbing intermittently.

I've seen the 2-business cards trick mentioned when advancing the pistons, does this work? Once you've removed them, do the pads and pistons not just advance further as soon as you brake hard?