Rear master cylinder

[alke46]

on my 2001 R1100RL.
Hope I don't wear out my welcome on this forum and also hope someone can answer a question regarding the rear master cylinder.

I have it all apart looking for anything that might be preventing it from moving fluid to the rear caliper. My question in particular is concerning the O-ring. As you can see in the attached photo, the piston has a groove in it that measures just under 8 mm and the O-ring has an inside diameter of just about 12 mm.

it seems to me that the O-ring should fit snugly in the groove on the piston. What do you think? And also can I source just an O-ring that would work?

Thanks for reading this and I hope someone can steer me in the right direction.

IMG_20220825_210102761 by Larry Alkire, on Flickr

 

[senseamidmadness]

You can still get the rebuild kits for these master cylinders from a few places as Brembo part number 110.4362.41, which is your best course of action as it'll include the other parts you need too.

That O-ring isn't the primary sealing surface that pushes the brake fluid -- the black rubber "apron" on the piston is, next to the spring. The O-ring is meant to fit to the outside diameter of the piston and the inside diameter of the bore, not that groove in the piston; you install it before putting the white spacer in, and it goes between the step of the larger counterbore and the white spacer. It doesn't move when the piston moves.

That's the O-ring that came out of it, so that's the size that has to go back in. Trouble is that the rings can swell over time (as you probably know, brake fluid is nasty stuff) so any measurement you take off that used ring is not guaranteed to be accurate. I can tell you that it's very likely made of EPDM rubber because that's typical for brakes.

I'm usually all for cheapskate tactics to make owning a bike cheaper, but brakes are probably the worst place to screw around trying save money. If you're rebuilding the master cylinder you should replace all the parts that have been worn from use or exposed to brake fluid and not just one O-ring. The Brembo kit is $27 plus shipping from this Ducati store and that's the option my cheap self would pick.

Edit: I just noticed you're looking at it because of "anything that could be preventing fluid from moving to the rear caliper". If you still have the original rubber brake lines on your bike, those are your most likely culprit. Rubber brake lines are not meant to survive 20+ years in any application, especially not motorcycles, and these bikes are old enough that any remaining original lines are flat-out dangerous.

Personally I did use a dirty cheapskate tactic on my non-ABS 1997 R850R by buying universal braided stainless ATV brake lines made by a company called Streamline, instead of the expensive Galfer or Spiegler lines (even though the Spieglers are worth every penny). They don't fit quite perfectly length-wise, but they have standard 10mm banjo fittings on each end and they work great. Sometimes the Streamline lines go on sale on Ebay for peanuts and I think I got all three lines I needed for about $40. If you have ABS though, generic lines aren't an option.

 

[alke46]

Thanks for the reply. I will order the kit from the Ducati store you linked me to.

As for the brake lines, I just got through installing new stainless steel braided lines.

 

[m_stock10506]

Thanks for the reply. I will order the kit from the Ducati store you linked me to.

As for the brake lines, I just got through installing new stainless steel braided lines.

There are plenty of these rear master cylinders available on eBay used. Also, the plumbing of the rear brake circuit makes it very difficult to get fluid flowing in the line. If you still have the ABS2 module, it is the highest point in the rear circuit. You will never get the rear master to pump fluid into the line just by pumping the foot lever. Anytime I have had to open the rear circuit and lost fluid pressure, I have back-filled the circuit by pushing brake fluid into the rear caliper bleeder until fluid starts filling the master. Even then, I have sometimes had to put a vacuum on the rear caliper bleeder after that, pump the foot lever until I finally feel the master start pumping fluid.

Personally, I wouldn't bother trying to rebuild the rear master cylinder.

 

[alke46]

There are plenty of these rear master cylinders available on eBay used. Also, the plumbing of the rear brake circuit makes it very difficult to get fluid flowing in the line. If you still have the ABS2 module, it is the highest point in the rear circuit. You will never get the rear master to pump fluid into the line just by pumping the foot lever. Anytime I have had to open the rear circuit and lost fluid pressure, I have back-filled the circuit by pushing brake fluid into the rear caliper bleeder until fluid starts filling the master. Even then, I have sometimes had to put a vacuum on the rear caliper bleeder after that, pump the foot lever until I finally feel the master start pumping fluid.

Personally, I wouldn't bother trying to rebuild the rear master cylinder.

I don't understand this statement. Why not rebuild? After all, it is 21 years old. If buying used off ebay, who knows whether it is any better than what I have now?

edit: non ABS model.

 

[oldcamper]

On this master cylinder design, are almost certainly two holes under the plastic angle fitting that connects to the remote fluid reservoir. Both holes are drilled through to the piston chamber. The one closest to the brake is very small and easily overlooked. It must be clear or the master cylinder won’t work. The only way to be sure is to remove the plastic angle fitting and verify. New fittings and the special o-ring that keeps it in place are available.

 

[alke46]

On this master cylinder design, are almost certainly two holes under the plastic angle fitting that connects to the remote fluid reservoir. Both holes are drilled through to the piston chamber. The one closest to the brake is very small and easily overlooked. It must be clear or the master cylinder won’t work. The only way to be sure is to remove the plastic angle fitting and verify. New fittings and the special o-ring that keeps it in place are available.

So does that plastic angle fitting just pull out? I wasn't sure about it and didn't want to screw something up by pulling hard on it. What about the availability? Dealership or some other after market seller like ebay?

Thanks for your response.

 

[oldcamper]

So does that plastic angle fitting just pull out? I wasn't sure about it and didn't want to screw something up by pulling hard on it. What about the availability? Dealership or some other after market seller like ebay?

Thanks for your response.

The plastic fitting can be pried out but the rubber may get damaged in the process. It’s in there pretty tight! Bevel Heaven sells replacements for $13. It may be easier if you warm it up a bit first.

That 2nd, tiny hole is often what gets plugged by old crud. It is very easy to miss but critical to master cylinder operation.

 

[senseamidmadness]

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I don't understand this statement. Why not rebuild? After all, it is 21 years old. If buying used off ebay, who knows whether it is any better than what I have now?

edit: non ABS model.

I agree with you -- the parts are still available and used replacements are never guaranteed to be in better internal shape. Unless the insides of your particular master cylinder are terribly corroded or damaged it's best to rebuild.

OldCamper is also right -- that angle fitting could be hiding something. The standard rebuild kit doesn't come with the parts for that area, but if you have a good set of calipers you can pop it out and measure the O-ring seats and the bore for replacement rings and get a reasonably close thickness measurement from the existing O-rings. It'll also be good to clear out those holes. Replacing those rings with generic equivalents is less risky as they're on the "no-pressure" side of the master cylinder, so if they leak they won't keep you from stopping unless literally all of your brake fluid leaks out.

Non-ABS gives you a pretty cool advantage in bleeding the system here: you can remove the master cylinder from its mounting point while leaving the caliper attached and the brake line intact, and lift it up towards the seat to persuade any sticky bubbles to migrate upwards.

Edit: Hey, good on you OldCamper for answering the question while I was typing! And the same store I linked even sells the part. How convenient.

 

[alke46]

Success

Got the rear m/c working properly today after removing that plastic elbow -- thanks for the tip of heating it first. After getting it out I noticed a lot of crud inside it so I cleaned the crud out and used tiny wire to make sure the small hole was clean and not blocked with crud.

Put it back together and it took a bit of playing around with it but I finally got it to fill the line and caliper and was able to bleed it. The front is still not working yet but I was so excited to finally be able to stop a moving bike, I took it out for a 55 mile ride just using engine braking and rear brake to stop.

Now I can tear apart the front master and I'm pretty sure it is gummed up as the fluid in the reservoir looked like motor oil when the bike came to me.

It's a work in progress, but I'm making headway.

Thanks for all your feedback.