18' KTM Front Brake Master Cylinder upgrade???

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12/30/2018 1:20 PM

So im not even close to happy w the front brake on the 18 250sx I just got. The lines and caliper and rotor are fine but its got the typical cheap Chinese master cylinder squish that a lot of new road superbikes are coming with these days. Like my new gsxr 1000 was similar level of junk so swapped it for a Brembo rcs 19 and that fixed the lever feel and reduces the squishiness and inconsistencies of it.

I saw a Supermoto rmz450 at the local shop that had a 16x19mm bamboo master and the lever didn't even move, it just worked on pressure instead of any amount of level travel, not looking for quite that much but it would be better than what it is now.


Ideas?


*note, its bleed just fine and yes I know its normal and you probably think its the best system out there but it just isnt sorry

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12/30/2018 2:25 PM

18 sx/SXF will have air in by this time. If you have to turn the adjuster in for the lever not to hit your fingers you have air. Common, it places air in two criritt places that can't be bleed the normal way.

Lever should be fully engaged maybe 15mm in, otherwise, air.

Had my brand new 2017 sit for 2 month in the garage with 8h on it, got back, brake was unusable. Bleed it, and as good as new. Can also happen if you lay the bike over so air slips in.

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12/30/2018 2:30 PM

I thought ktms come with brembos

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12/30/2018 2:38 PM

kb228 wrote:

I thought ktms come with brembos

They do.

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12/30/2018 2:54 PM
Edited Date/Time: 12/30/2018 2:55 PM

I changed the fluid to motul rbf600 regardless on all bikes so I bleed out the crap that was in there. Its the way its "supposed to be" just not good enough for me. It looks the front wheel w 1 finger on the closest to bar reach setting, it just has too much squish as is. Needs different piston ratio.
what other master cylinders are people running? just supermoto stuff?

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12/30/2018 3:03 PM

try a Nissin from a late model CRF450.

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vomiting equals disqualification.

12/30/2018 3:16 PM
Edited Date/Time: 12/30/2018 3:16 PM

You can get an older SXF reservoir from 2012-2014, it has 10mm piston instead of 8 or 9mm that is on the later models. Not sure exactly when they switched and what earlier has 10. I like the current better, much more modulation. Used to run 10 and 270 but was to aggressive in ruts.

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12/30/2018 3:32 PM

Tie the front brake lever all the way into the throttle for a few hours and it will be rock hard. Mine does it if the bike sits for a week or so and I wrap the tie down around the lever on the way to the track and it’s good to go. Not sure why it does, but I have no external leaks and no air in the lines.

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2018 KTM 250sx
Instagram CamaroAJ

12/30/2018 3:48 PM

AJ565 wrote:

Tie the front brake lever all the way into the throttle for a few hours and it will be rock hard. Mine does it if the bike sits for a week or so and I wrap the tie down around the lever on the way to the track and it’s good to go. Not sure why it does, but I have no external leaks and no air in the lines.

this is an old road racing trick as well to zip tie it over the week to get all the flex out of the lines etc and then tap the caliper and line all the way back up to res with a light wrench to help facilitate if there is a stuck bubble anywhere.. did that already of course.

Illl have to check out the prior year master to swap

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12/30/2018 5:44 PM

aees wrote:

You can get an older SXF reservoir from 2012-2014, it has 10mm piston instead of 8 or 9mm that is on the later models. Not sure exactly when they switched and what earlier has 10. I like the current better, much more modulation. Used to run 10 and 270 but was to aggressive in ruts.

The '18 250 SX has a 10 mm. They went from a 9 mm to 10 mm on the SXF in 2014.

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12/30/2018 7:48 PM

There isn't an upgrade for the master cylinder that comes stock if it's working properly. You can buy a caliper from KTM Power Parts that is better.

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Take it to the limit, one more time!

12/30/2018 8:35 PM
Edited Date/Time: 12/30/2018 8:37 PM

mx317 wrote:

There isn't an upgrade for the master cylinder that comes stock if it's working properly. You can buy a caliper from KTM Power Parts that is better.

It is 100% not caliper flex that I’m talking about, outright power and modulation is also not an issue at all, it’s the lever ratio. Needs to move more volume at higher lever pressure for reduced finger travel.
The rear brake lever action is perfect. Have it set so there’s maybe 1mm of free play then it’s enguaged and it hardly moves

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12/30/2018 10:11 PM

mx317 wrote:

There isn't an upgrade for the master cylinder that comes stock if it's working properly. You can buy a caliper from KTM Power Parts that is better.

GCBC wrote:

It is 100% not caliper flex that I’m talking about, outright power and modulation is also not an issue at all, it’s the lever ratio. Needs to move more volume at higher lever pressure for reduced finger travel.
The rear brake lever action is perfect. Have it set so there’s maybe 1mm of free play then it’s enguaged and it hardly moves

I know what you are saying, try the old SX master cylinder that some have suggested. Also the zip tie trick does tighten things up.

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12/30/2018 10:14 PM

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12/31/2018 9:49 AM

Don't trust eBay listings for fitment. There were different bore sizes though out those years. The '06-'08 were 10mm, the '09-'13 were 9mm, and the '14-'19 are 10mm. You have to go back to 2004 for an 11mm. I'm not sure about lever ratios... I think the new stuff is 15mm.
There's a lot of info here:
https://www.oppracing.com/category/1012-brembo-oe-oe-front-brake-master-cylinders/

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12/31/2018 6:03 PM

What is the negative effect of too big piston? Too much pressure and on off feel vs smooth?

What is the stock 18-19 clutch master piston, any negative in upgrading to larger piston?

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12/31/2018 8:43 PM

CarlinoJoeVideo wrote:

What is the negative effect of too big piston? Too much pressure and on off feel vs smooth?

What is the stock 18-19 clutch master piston, any negative in upgrading to larger piston?

Big pistons gives a more on/off feel. 5psi on a small piston is a lot less force than 5 psi on a larger one. Theres a sweet spot for good modulation and power when you need it. What that is - i dont know. Never had the luxury of trying diff sizes.

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12/31/2018 8:50 PM

For me the negative is increased lever effort. I like the feel at the lever of the Brembos and think that the light lever effort is easier to modulate, especially if I have a little arm pump.

Actually, you have to consider the entire system starting with the force at the lever. A larger master requires more force on the lever to produce the same braking power, everything else being equal. The new Brembo stuff ('14-'19) uses a 10 mm master with 24 mm caliper pistons. If you do the math, that's a hydraulic ratio of 11.52:1. The '09-'13 KTM brakes used a 9 mm master with 24 mm calipers, i.e., a hydraulic ratio of 14.22:1. So, for the new stuff the force applied at the master is multiplied 11.52 times and for the '09-'13 brakes the force is multiplied by 14.22. The larger the master (for a given caliper size) the less line pressure (braking force) for a given lever effort. The trade off is that the higher the hydraulic ratio the greater the distance the master cylinder piston must move to develop a given pressure. You can apply the same principle to the clutch.

Interestingly, when Andrew Short was on KTM in '13 he wanted a stronger brake for outdoors, so he used the '08 master (10 mm) and '08 caliper (28 mm) for a 15.68:1 hydraulic ratio.

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12/31/2018 9:12 PM
Edited Date/Time: 12/31/2018 9:13 PM

kb228 wrote:

Big pistons gives a more on/off feel. 5psi on a small piston is a lot less force than 5 psi on a larger one. Theres a sweet spot for good modulation and power when you need it. What that is - i dont know. Never had the luxury of trying diff sizes.

The on/off feel was especially apparent on my RMZ when I went to an oversize rotor. The power was now there, but it lacked modulation. And that brake always had a hard (not mushy) lever. When I replaced the caliper with a Ride Engineering caliper with bigger pistons the feel and modulation became identical to my KTM. That's not surprising since it has nearly the same hydraulic ratio (11.39:1 vs 11.52:1).

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12/31/2018 9:22 PM

So bigger piston doesn’t just directly mean easier pull at the lever? I see a 10mm clutch on that site and was wondering if that would be an easier pull.

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12/31/2018 9:33 PM

CarlinoJoeVideo wrote:

So bigger piston doesn’t just directly mean easier pull at the lever? I see a 10mm clutch on that site and was wondering if that would be an easier pull.

No, bigger master harder pull.

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1/1/2019 6:05 AM

Good info in this thread

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Take it to the limit, one more time!