Uh oh, leaking rear master cylinder - Wrist Twisters
 
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post #1 of 20 Old 07-23-2014, 05:25 PM Thread Starter
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Uh oh, leaking rear master cylinder

I ordered new OEM brake pads for the front and rear of the 919 yesterday. I guess the rest of the brake system is jealous. This morning after riding to work, I noticed a drop of brake fluid coming out of the rear master cylinder boot.

I have a piston and seal kit coming in the mail now. If anyone is interested (yes? no?), I'll take a few pictures and post as I do the repair. It's not a major ordeal to spruce up an old cylinder, but some folks haven't done it before.

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post #2 of 20 Old 07-23-2014, 05:28 PM
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I'd love to see some pics! You never know what you'll run into....

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post #3 of 20 Old 07-23-2014, 05:35 PM
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There's a rear brake? :what:

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post #4 of 20 Old 07-23-2014, 05:45 PM Thread Starter
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Quote:
Originally Posted by crimedog View Post
There's a rear brake? :what:

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post #5 of 20 Old 07-23-2014, 06:34 PM
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Hell yeah, post it! Thanks for taking the time to do it.

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post #6 of 20 Old 07-26-2014, 10:45 PM
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Not the same caliper, but due to a stuck pad, I had to tear apart the rear caliper on my T. It might help:


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post #7 of 20 Old 07-27-2014, 08:58 AM Thread Starter
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Might help, yeah. But I'll be working on the master cylinder, not the caliper.

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post #8 of 20 Old 07-27-2014, 09:15 PM
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Yeah, I should have taken some video of the master apart, and probably a close up of the lithium grease I used. I've had a lot of requests for just that part of the video that I didn't put in.

2009 Aprilia Tuono - Ginger
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2006 Honda 599 - Ex wrecked it :-D
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post #9 of 20 Old 08-02-2014, 10:40 PM Thread Starter
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And so, here's how it went:



The master cylinder was leaking a little fluid past the rubber boot at the bottom. It also wouldn't stay bled - go figure.






This is the piston kit for rebuilding the master cylinder. It's a lot cheaper than a new cylinder, so if the cylinder itself is in good shape, you can go this route.




The first order of business is to drain the reservoir. A turkey baster works well for that, or you can unscrew the reservoir and pour the fluid out. Then remove the reservoir tube and the brake line from the master cylinder. Be sure to protect the paint around the area from the brake fluid.




This is the end of the master cylinder, with the plunger still held in with a snap ring. You need a pair of snap ring pliers to remove the snap ring.




Here are the parts as they came out of the master cylinder.




In this case, the master cylinder bore was in great shape, so I cleaned the little bit of gunk out. This is an aluminum cylinder, so there's not a lot you can do if the cylinder is scarred or scuffed. You could try to polish it up, but it's less likely that you will get a leak-free assembly than if this was a steel or iron cylinder.




This is the grease I used to lubricate the new piston seals. This is DuPont Krytox GPL206 fluorinated grease - basically teflon particles in a fluorinated oil. This grease is stable across a huge temperature range, and it does not react with brake fluid at all.




Putting the master cylinder back together is very straightforward. The piston and seal set go into the bore easily, and the snap ring seats without much of a fight.




Don't forget to put the new rubber boot on the plunger before putting the rearset hardware back together.




Finally, bleed the system however you prefer. I've had good success with both the reverse bleed method, and the conventional bleeding from the reservoir through the calipers.

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post #10 of 20 Old 08-03-2014, 04:15 AM
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Nice write-up. Looks pretty straightforward. Thank you!

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post #11 of 20 Old 08-03-2014, 04:40 AM
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Quote:
Originally Posted by marylandmike View Post
Nice write-up. Looks pretty straightforward. Thank you!
+1, thanks for sharing!

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post #12 of 20 Old 08-06-2014, 01:40 PM
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Mine isn't leaking from what I can tell, but it won't stay firm throughout the riding season. Come spring for the last two years, I've needed to bleed to get it back.
I'm thinking this kit will be the ticket, no?
It all comes down to the condition of the bore. I won't know 'till I tear it down, but I'm not doing that 'till winter.

I plan to die young, as late as possible.
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post #13 of 20 Old 08-06-2014, 02:23 PM Thread Starter
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Quote:
Originally Posted by semi_gray View Post
Mine isn't leaking from what I can tell, but it won't stay firm throughout the riding season. Come spring for the last two years, I've needed to bleed to get it back.
I'm thinking this kit will be the ticket, no?
It all comes down to the condition of the bore. I won't know 'till I tear it down, but I'm not doing that 'till winter.
It's a really easy repair to make, so if you take the cylinder apart and the bore looks good, installing a new piston kit is a no-brainer.

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post #14 of 20 Old 08-06-2014, 04:20 PM
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Quote:
Originally Posted by MF1VE View Post
It's a really easy repair to make, so if you take the cylinder apart and the bore looks good, installing a new piston kit is a no-brainer.
Yeah, I've got no issues doing the job. I was just wondering out loud what I might expect the bore to look like.
I've kept up with fluid flushes, so I don't expect much of a problem.
It's just that every time I seem to try and save a buck by buying a part of a part rather than the whole thing, it blows up in my face.
I'm just lucky that way.

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post #15 of 20 Old 08-06-2014, 06:21 PM
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Quote:
Originally Posted by MF1VE View Post
It's a really easy repair to make, so if you take the cylinder apart and the bore looks good, installing a new piston kit is a no-brainer.
Hey, where'd you get that kit? It seems Bike Bandit doesn't have them in stock.

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post #16 of 20 Old 08-07-2014, 09:37 AM Thread Starter
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I think it was cheapcycleparts.com

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post #17 of 20 Old 08-07-2014, 01:40 PM
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Great! Order placed.
Thanks.

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post #18 of 20 Old 09-08-2014, 01:59 PM
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Just to wrap this up:

I installed the kit over the week-end and went for a ride.
Excellent results!

I shopped for the Krytox lubricant MF1VE mentioned, but it was very pricey.
Instead, I went with WILWOOD 290-11087 Synthetic Brake Assembly Lube.
I picked this up on Amazon.

After re-assembling the piston, I put a dab of silicon grease which is a bit heavier than the assembly lube at the rounded end of the plunger (outside the brake fluid area, but within the boot).

I re-adjusted the pedal placement a bit lower, then needed to re-set the brake light switch since I had reduced the pedal travel a fair amount.

Now I know there are some of you who don't use the rear much if any and I rode without it for a couple of weeks, but I'm real glad to have it back. Especially at parking lot speeds.

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post #19 of 20 Old 09-08-2014, 04:11 PM
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Quote:
Originally Posted by MF1VE View Post
This is the grease I used to lubricate the new piston seals. This is DuPont Krytox GPL206 fluorinated grease - basically teflon particles in a fluorinated oil. This grease is stable across a huge temperature range, and it does not react with brake fluid at all.


Just looked up this grease as it is something I would consider for Tuono's clutch slave piston. $60 for 2 oz tube???????

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post #20 of 20 Old 09-08-2014, 06:23 PM Thread Starter
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Originally Posted by zaq123 View Post
Just looked up this grease as it is something I would consider for Tuono's clutch slave piston. $60 for 2 oz tube???????
Yep! It's some magical stuff though. I buy about one 2oz tube every 2 years or so. We use it a lot at work.

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