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Discussion Starter · #1 ·
Hi, I'm using a GSX-R front master cylinder on another bike and it needs rebuild as I can't get it to bleed firm. It sometimes stays firm and other times the lever gets pretty close to the grip. MC casting mark is 11/16. Anybody know what the right kit is, or what year/model it came from?
 

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Post a pic here. As a wild guess, 11/16 might be the piston size. Is there a bleeder on it? It would help if you'd measure the bore diameter. How do you know it's off a GSX-R?
 

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Discussion Starter · #4 ·
Post a pic here. As a wild guess, 11/16 might be the piston size. Is there a bleeder on it? It would help if you'd measure the bore diameter. How do you know it's off a GSX-R?
Piston measures 0.686, bore 0.688 as near 11/16" as you can be. It was free, and I was told it came off a 2006-2007 GSX-R600. Can't verify.

That said, the OE axial MC is 15mm and this being more than 17mm probably throws the MC/caliper ratio off, although it felt just fine in use. I have the OE MC on until I can figure this out anyway. On a 2014 MT09 with dual 4 pot radial calipers if that matters.

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The K6 750 service manual says the MC has a ~19 mm bore while the K8 750 says ~17.5 mm. I'm assuming that the 600's are the same. So you might have the K8 part, which is pn 59600-01H20. There are pics of one here. I didn't see 11/16 on it but they don't have a good pic of the area where your pic indicates that it should be. A snag here is that K8 is right in the middle of the MC recall campain. A recalled MC would have the same bore but the rebuild kit might be different. You'll have to check to see if the recall actually affected it. Non-recalled MC's are relatively rare as Suzuki wanted them back as part of the recall.

The K8 caliper has ~30.3/32 mm bores. If you know the bore diameters of your current calipers, you could calculate the hydraulic ratio. Ideally it should be about 27. Off hand, the 19RCS sounds too big but the 17RCS might be near ideal.
 

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Discussion Starter · #9 ·
The K6 750 service manual says the MC has a ~19 mm bore while the K8 750 says ~17.5 mm. I'm assuming that the 600's are the same. So you might have the K8 part, which is pn 59600-01H20. It looks like this:

The snag here is that's right in the middle of the MC recall campain. A recalled MC would have the same bore but the rebuild kit might be different. You'll have to check to see if the recall actually affected it.

The K8 caliper has ~30.3/32 mm bores. If you know the bore diameters of your current calipers, you could calculate the hydraulic ratio. Ideally it should be about 27. Off hand, the 19RCS sounds too big but the 17RCS might be near ideal.
Thank you for this info. I'll get on my supplier and see if I can get the right kit.
 

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^You responded while I was updating my post. The update has more/better pics. The feeling is termed "wooden" if the MC is too big.
 

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Wouldn't that push the ratio too far and make the brakes like an on/off switch?
In simple terms if you use a 15 mm the master will make more pressure less volume and lever will come closer to bars . 17mm or 19mm you wont have to move lever as far. It’s a feel thing and personally I love how the 19mm works. I have them on both of my bikes. Also it’s a radial master where as yours isn’t. Win win as far as feel. Also has a bleeder at top which makes it very easy to bleed .
 

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Discussion Starter · #12 ·
^You responded while I was updating my post. The update has more/better pics. The feeling is termed "wooden" if the MC is too big.
Thanks. This is what the MC and caliper bores are specified in the SM. Not sure what the 30.23 and 27 refers to tho, aren't they all the same? Or is there a size difference for the leading/trailing piston on each caliper?:

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Riding today with the 15mm MC vs the 17, effort seemed similar but travel longer. Hard to do an A/B comparison when the radial MC is not firm though. The radial definitely feels better on the street, and was great on the track despite the random lever gap. (And no, I never felt the brake was going to 'fail', despite how hard I was using it).
 

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Discussion Starter · #13 ·
In simple terms if you use a 15 mm the master will make more pressure less volume and lever will come closer to bars . 17mm or 19mm you wont have to move lever as far. It’s a feel thing and personally I love how the 19mm works. I have them on both of my bikes. Also it’s a radial master where as yours isn’t. Win win as far as feel. Also has a bleeder at top which makes it very easy to bleed .
I do get the mechanical advantage you get with smaller/larger bores on the MC.
My GSX-R1000 has a Brembo 19 and it gives stellar feel.
 

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The caliper bores are commonly different and the service manual specs should say leading & trailing. They do in every manual that I have. A 15 mm master sounds too small and will tend to give a spongy feel. But the calcs will tell all. Suzuki switched to radial masters about 2004-2005. The one in your pics looks like a radial. What year/model is your manual? I hope it isn't a generic Clymer manual.
 

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Discussion Starter · #15 ·
I'm using the factory SM (Yamaha FZ09) which lists the MC as 15mm, calipers 27/30.23, 4 pots of each size (5161.2 mm2). My math puts the ratio at 29.2 with the 15mm factory axial MC, and at 21.55 with the 11/16" Nissin.

For clarity, this is a 2014 FZ09 with OE calipers, and I happened to have a 'free to me' 11/16" Gixxer radial MC to play with.

This is all newish territory for me as my background is in cars and I've never played with MC/Caliper ratios, just trusted what the factory installed. Plus I'm pushing 60 and rode my first motorcycle in March/21. I have a K6 Gixxer 1000 too.

PS: Clymer, lol :)
 

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The 11/16" Nissan is too big for those calipers. I think that the 2008 YZFR6X has a 16 mm MC but wonder if you'd notice the difference. The K5/K8 GSX-R1000 has a 19.1 mm MC and an inferior 22.5 ratio. You can improve it by switching to a 11/16" MC as was used in K9/L1.
 

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Discussion Starter · #17 ·
I appreciate the help with this. For the marginal potential benefit of a radial MC, I think I'll err on the side of leaving it be and keep the axial 15mm OE one. I've already spent way more on this machine than can be justified for running 6000km/season.
 
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