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

buckmulligan

I posted this over in the Road Workshop section but I hope it's not frowned upon to repost it here, because it's a somewhat urgent situation and I'm hoping you guys can help a bit more seeing as hydraulic discs are more a MTB area of expertise!

I've 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!

The Rookie

Yes you can, the internals are effectively the same with critically the same piston diameter, will work just fine.

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!

timothyw

I caveat this that I have no direct experience of hydraulics in any form, but I'm sure I read someone in another thread saying that yes, you just need to remove a little bit of fluid from the system.

The Rookie

Letting out fluid will have no effect unless you overfilled it in the first place, but if the Pistons were retracted and have come out at all, that's unlikely.

A bit of rub of pad on rotor is perfectly normal, to work disc brakes run very small pad clearances.

I hope the original issue didn't stem from a little pad rub?

buckmulligan

Thanks for the reply.

I bled the system as per Shimano's instructions, by pushing the pistons back and inserting a bleed block, injecting fluid at the caliper to a funnel at the lever and then locking it off at the caliper and finally replacing the screw at the lever; it was completely full to the top with mineral oil as I replaced the screw there. The lever had a nice solid feel with the bleed block in place, so I cleaned everything up, replaced the pads and wheel and then pumped the lever until the pads started to bite the rotor; this took maybe 3 full lever pulls? Is this the correct procedure? Could that have led to it being overfilled and/or the pistons/pads being too far advanced?

I appreciate that they run with tight tolerances, but it seems like these are way too tight! It was running with zero-rotor rub with the old caliper, the rotor is very true and was hoping that I could achieve the same with the Deore caliper.

The Rookie

If you can spin the wheel freely the rub isn't doing anything, if it's really binding then it's an issue.