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3/1/2016
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My front brake started feeling weak a while back. I like my brake to be pretty touchy, which it was when the bike was new. First thing I did was replace the pads. When that didn't do it, I went to a Core Moto braided line and Motul 600 fluid. Then I replaced the rotor. I've bled them many times and have tried the zip tie trick on the lever many times - still doesn't have much stopping power on the track.
Should my next move be to replace the master cylinder? Caliper?
Get a 19+ CRF master cylinder and a 2007 YZ450F Caliper assembly.
Prior to replacing the caliper or master cylinder, I would try to rebuild both of them. Of the two, the master cylinder would be my first priority.
Sometimes these seals go bad and cause weak brakes, even if a fluid leak isn't visible. The rebuild is quite easy and there are a plethora of videos on the process.
Should be, according to Partzilla:
5XC-W0041-00-00 for front master cylinder rebuild kit, about $39.
5XC-25802-00-00 for the front caliper seals and pistons, about $55
or
5KM-25803-10-00 for just the caliper seals, about $13
I've heard of people doing things like this. Is there a particular reason? Also, what bugs me is that it worked pretty well when it was new, then got weaker later.
I think I'll be trying this first
The Shop
Luxon 4-Post Bar Mounts
$189.95 - $239.95
DeCal Works Huge Plastic Inventory of UFO and Polisport kits.
You reverse bleed with a syringe or through the bleeder?
Check the slide pins on the caliper for any grooving or bends, I assume since you’ve replaced the pads you replaced or inspected the pad pin as well.
The 19+ CRF master cylinder has a bigger piston. It moves more fluid than the YZ MC (which I believe is a 9mm versus the 11mm CRF).
The 07 YZ450F caliper has bigger pistons than the newer one.
I just did this mod, along with the core steel braided brake line on my 23 YZ450FX and the feel at the lever is much better. The power of the front brake is better and to me feels more modulated. This is all using the stock brake rotor.
I put a KTM brembo master cylinder and my brothers 2021 YZ 250 and he has yet to complain about the front brakes. Keefer has an article on it.
The Brembo MC is all you need the caliper is fine. I have the Brembo on my 2021 YZ250f and a Honda MC on my wife's 2019 YZ250f. They're both a big improvement over the Yamaha one but the Brembo unit has amazing feel and you can really modulate it where the Honda just has more power.
Here’s the article with the part number.
https://keeferinctesting.squarespace.com/motocross-testing/2019/3/28/to…
If you want to fix this problem once and for all, I've got a Ride Engineering V2 caliper with Honda master cylinder and ARC Memlon lever, using a CoreMoto stainless steel brake line and MotoStuff 280-mm rotor plus relocation bracket. Strongest, most progressive brake setup I've tried yet. It also has OEM Honda brake pads.
The system is like new (maybe 5 hours total) and is bled and ready for you to install.
PM me if interested.
We put on a 22 Honda Master on our 19 YZ250F made it stop so much better. The feel at the lever went from pulling 1.5cm to get braking to instant brakes when you breathe on it. The stock master after 80-100 hours gets worn out loses a ton of power.
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