Bleeding the rear Shimano hydraulic brake
 

Hi there,
I want to convert my tandem from mechanical to hydraulic brake systems. I know that the length of the brake hose can be extended using a hydraulic line coupler. However, is there a commercially available bleed kit which comes with syringes and reservoirs that are large enough to carry sufficient mineral oil to the job, and is compatible with Shimano? Specifically, I'm having a Shimano BR-RS785 caliper. Thanks!

I would never install hydraulic discs on a bike that didn't have them OEM. As a result, I don't own any bikes with hydraulic discs, but I understand them. Questions like in the o.p. mean that this job should be taken to a pro. These are your brakes! I would not think that special reservoirs or syringes exist for tandem rear brakes. They might, but the additional fluid capacity resides in the brake line. IF a single syringe load of fluid could not fill the entire system (and it probably could not if totally dry to start) then as many re-fills as necessary would be the procedure. My friends at the LBS like to joke about tandem people and the kinds of money that can be made selling them special purpose kit that they don't always especially need. Likewise, the lever/caliper fitments (reservoirs, master cylinders, etc.) will not be different for tandems because when the brake lever is activated, the master cylinder only 'sees' (hopefully) a pressurized brake system. The length of the brake line is irrelevant, all other things being equal (properly extended, bled, routed). Given that rear brakes (even on tandems) have so little to do, I would give LOTS of thought to leaving the rear end mechanical. I've never done it, but to properly bleed the rear caliper, the bike has to be vertical. I store our tandems vertically in our townshouse 'garage'. I know what is involved in getting a tandem vertical. Even at the LBS it won't be a walk in the park. All of this FWIW.

I converted our CoMo from mechanical to SRAM red hydraulic brakes and the standard SRAM bleed kit was fine. That doesn’t answer your question but the conversion was easy and the results were greatly improved breaking. This was my first foray into hydraulic and it was very straight forward. There is not a lot of additional fluid required for the extra hose length and there should be extra capacity in a bleed kit. I know that Park Tool sells a bleed kit but don’t know much about it. You may also check Hope or Magura to see what they offer. Good luck.

I use a regular Shimano bleed kit with XT calipers with no problem.

it’s not Rocket Surgery. If you can do basic bike maintenance you can learn to work on hydraulic brakes, particularly with all the info available on you tube.

and you don’t need the bike to be vertical. The caliper just needs to be the lowest part of the system, and the lever rotated to the correct angles during the process. I’ve watche professional mechanics bleed brakes, and done it myself, and nobody ever put the bike vertical.

getting some slope on the run is the hard part. I take the caliper off so I have some angle. but the regular beed kit is fine.

Converted from Mechanical calipers to hydraulic maybe 5 years ago. Best upgrade to our CoMo Mocha I have made to date. and no problems at all.
If you foresee any situation where you will be dragging the brakes, go with heavier downhill MTB 4-piston calipers. These will work with same-manufacturer road levers. I use the RS-785 shimano DI2 brifters and SAINT 4 piston calipers. My rotors are full float Magura 203's designed for e-bikes. The pull is a little longer on the brifter, but never bottoms out. The SAINT calipers can be paired with matching 3 meter long hose + Ti-Cycles hose coupler + RS-785 hose, again with no problems.

The last thing you want to do is overheat the caliper and lock up the pistons or boil the fluid. The larger mass of the SAINT system and heavy duty rotor seems to prevent this.

Bleeding brakes on a tandem is pretty much the same process as with a regular bike. Helps to have a tall-ish bike stand, and maybe a step stool, to get a good angle for the line. It doesn't need to be vertical, just angled enough to let the bubbles rise. If necessary, remove the caliper to get a steeper/smoother angle. If you can't purge all of the air the first time, or if the oil is still dirty when flushing old brakes, just repeat the process until you're happy.

As for whether it's worth it to install hydraulic brakes, that's really a matter of personal preference. I have hydros (Magura MT4) on my MTB tandem, which make it a lot easier to handle on really steep descents without my hands going numb. But I have mechanical brakes (Avid BB7) on my road tandem, which work surprisingly well--even on 3000'+ 5-15% descents in Colorado. Both systems are easy enough to maintain, and neither has ever failed. I do recommend installing the largest rotors that you can fit, regardless of the system.

I take issue with the earlier comment that the back brake doesn't do much. In my experience, the back brake pads wear just as much as the front--which is a clear indication that they are being used equally. While braking hard, especially if the surface is wet/dusty/loose, using more of the rear brake and letting up on the front can prevent the front wheel from washing out and causing a crash.

Like many, I've always been worried about losing my brakes. After extensively researching and testing braking techniques, I've found that there are three primary considerations: 1) Brakes cool down very quickly once they are released, especially with some speed to flush moving air over the discs. This is why "pulsing" the brakes is so effective, and you can even alternate front and rear brakes to enhance the cooling cycles. 2) Going fast increases wind resistance, which reduces the amount of energy that the brakes need to absorb. We call it using "air brakes." Descending slowly actually results in hotter brakes--although terrain like switchbacks can unfortunately force slower speeds. 3) Tandem brakes can get really hot, even with proper technique. But, after a lot of research, I've found that the failure rate of well-maintained disc brakes seems to be highly exaggerated and suspiciously undocumented--as in, "my friend's cousin's dad says that he heard about some people who had a buddy of theirs who knew a guy who lost his brakes." Because I'm paranoid, I still stop and let the brakes cool if they start to really chatter. But, even with discs discolored with heat, I've never lost my brakes. Here's a great video demonstrating just how much abuse disc brakes can take, even with small discs and terrible technique:

Having used Shimano RS785 hydraulic brakes for quite a few years on our travel road tandem, I can report no show stopper issues, they provide much more braking power than cable versions and no loss of performance due to cable length/stretch/sponginess. We also use Shimano XT 4 piston hydraulic brakes on our full suspension mountain bike tandem.

I would also argue the comment about rear brakes not having much to do. I guess that depends how you decide to use your brakes. We can use the rear brake equal or more than the front brake in a lot of situations, even though we are somewhat handicapped by my stoker only weighing 93lbs (very little weight over the rear wheel). Especially on our road tandem, I prefer to not over-stress the fork by only relying on our front brake as one poster above seemed to indicate.

One point to note for the OP, since the RS-785 uses simple compression nuts & olives at both ends, you can source a single long length of hose to run to the rear caliper without any additional coupler needed. We do this with a single hose run even though our tandem is coupled (the tandem has hose clips which allow for the hose to be removed from the frame, and then dismount the rear caliper from the frame which allows for easy storage for travel. I opted to use BH90 hose instead of the default BH59 because I prefer the more solid feel that the BH90 produces at the levers (also requires a different size olive installed at each end. It's not rocket science to switch hose types of you wish... any decent LBS that handles mountain bikes or higher end road bikes should be capable if you need assistance.

One last point, we opted to use semi-metalic pads on all our tandem disc brakes which have much more consistent braking performance in both wet and dry conditions. They also last much longer than resin (or organic) pads. Downside is a little less immediate "bite", but an offsetting advantage is that they do not fade nearly as much as resin/organic pads under heavy load.

If your tandem frame cannot route hydraulic hoses neatly, another good option and a lot less money is to go with a hybrid solution such as the TRP HYRD calipers which are cable actuated hydraulics at the caliper location, and you can keep using your cable brifters without needing to upgrade those too.

As for bleeding the brakes, especially rear, if you store the tandem vertically with levers at the top, IMO that is positively the best assurance of any remaining air migrating to where it can be removed easily - at the shifters.