Master Cylinder - the bleeder

Gra900

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Hi All, I just fitted a master cylinder off an 2015 XBR 125, thinking it’s a single disc setup so should give good feel. It’s an 11mm piston. I’ve filled it up and bled about half a can of fluid through. No sign of bubbles at the bleed nipple. The calliper has been shop refurbished. However the lever still pulls back to the bars, though it does brake a bit. Got a quality braided hose.
Any ideas? I’m thinking either I need a bigger piston or I’ve still got air in the system.
Graham
 
And a couple pics
 

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I know the stock master has an adjuster on the lever to reduce free play. Yours seems not to.
 
I'm curious to see how this is resolved, I'll be bleeding lines this winter. Would the air-free line still being playable down to the grips mean the line itself is too long? That wouldn't make sense though with a hydraulic line though right?
 
My initial thought was that the MC doesnt push enough fluid for a big single piston caliper.

The CBR ( If thats what you mean ) has a dual piston caliper and the smaller pistons I would think produce more stopping force with less pressure.

Heres the MC I use. I like that it uses a pressure switch and works plenty good

https://yambits.co.uk/xs650-brake-master-cylinder-p-37409.html

The actual one I use... Has the DOT sticker on the cap,
https://www.mikesxs.net/yamaha-xs65...nder-oem-366-25850-02-00-366-25850-02-00.html
 
well I did my first ever attempt at bleeding the line tonight. Had everything set up. Squeeze, loosen, tighten, release. Worked good for a few pulls then went dead. The old brake fluid was so dirty. Got as much as I could out then removed the line. Barely any came out. I’m wondering if the line itself is clogged up somehow. The new fluid I put in was remarkably clearer than the old stuff. I’m not quite sure where to go next on this. Sorry to hijack the thread - any help would be appreciated.
 
Addison, You mention old fluid, Are you just trying to just change the fluid?
If you are using the stock rubber hose? These hoses are very old. They tend to rot from the inside out.
When doing brake work such as this it is often best to tear down the M/C and Caliper Clean and reassemble.
Replace that old hose with some thing new. Either a new rubber hose or a Braided Stainless steel hose(best option).
This gives you all clean parts.
Use fresh brake fluid to bleed this clean system.
Leo
 
XSLeo, thank you for the tips.
After work today I started over. I don’t want to screw this up, I need my brakes to work!

I stripped the master cylinder down completely and cleaned it inside and out. Took off the rubber diaphragm from the inside, cleaned the crud off, inspected the springs and piston and then cleaned it, blew compressed air through the entire unit which pushed out a surprising amount of shit, and I guess all that’s left is to replace the line.
 
On a hydraulic system the length of the lie isn't that important. When functioning properly the line is completely full. Brake fluid isn't compressible, so any fluid pumped into the full line the same amount is pumped out the other end.
In a brake system, pressure is what does the work. Volume is secondary.
The pads don't move very much a mm or two is about it.
In the stock system the 14 mm M/C and the stock caliper work, just not as well as they could. They tend to be a bit wooden. That's when you pull the lever and don't feel much braking, so you pull harder with little added braking till you pull so hard the brake locks.
As far as the caliper go. Line pressure is the controlling factor. Going to a smaller bore master cylinder increases line pressure. Thus going to a 12 or even 11 mm bore does improve braking and feel. This does slightly increase lever travel.
Another way to increase braking is to use a stock M/C and add a second caliper and rotor.
The M/C delivers the same line pressure but this is divided to two calipers. This doubles the braking force.
I have bikes set up both ways. The 12 mm and single caliper/rotor work very close to the same as the 14 mm M/C and dual caliper/rotor set up.
The dual set add a lot more unsprung weight to the front end. This extra weight does effect the handling response. Under most road use you won't really notice the difference. The more aggressive you ride the more noticeable the difference.
Leo
 
Hey Addison and Leo, great discussion
Update on 11mm XBR master cylinder. I have got it bled, eventually by removing calliper and moving it around to dislodge air. Brake feels excellent and progressive, but yet to try it on the road.
 
Hey Addison and Leo, great discussion
Update on 11mm XBR master cylinder. I have got it bled, eventually by removing calliper and moving it around to dislodge air. Brake feels excellent and progressive, but yet to try it on the road.
Same, yesterday I took my old jelled up line off, bought a new stainless line, and banjos, and washers. Pumped fresh DOT3 through it all and finally have a working front brake now. Bike is functional, so took it for a rip, love riding it!!
 
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