Bulging master cylinder repair

[kdxdazz]

This came up on my Facebook memory and thought I would share, kdx's are known for the bulging rear master cylinder reservoir problem, mine was so bad that when I would do a river crossing the rear brake would fail
I own 4 bikes and all had the bulging master cylinder problem caused by heat from the exhaust, this is the reason later model bikes went to an integrated reservoir system but some such as later klx250's mounted the reservoir higher up.
Just pop it in the oven for 10 minutes, job done

[Chuck78]

This has always troubled me that they bulge out in the middle like that... I was looking at converting to at a later integrated reservoir type rear master cylinder, but each manufacturer uses their own bore size on the rear masters, meanwhile nearly all fronts are 11 mm across manufacturers.... The integrated KX masters have a slightly different mounting tab location placing them at the wrong angle, and many or all of them put the banjo bolt right up against the frame which causes additional trouble if trying to align things with the caliper guard intended for your bike frame model.

I believe the Yamaha YZ/WR & Honda CRF/later CR style might be a good fit, or better than the Suzuki and Kawasaki versions with their rectangular reservoirs, but that would mean changing for a caliper from those bikes also, in order to keep the same hydraulic leverage ratio. Some older CR models use the same rear brake pads as the KDX so I'm fairly confident those calipers would swap onto the KDX caliper hanger bracket.


This warping of the rectangular plastic reservoir is likely also why Kawasaki KX models use a round screw cap on the remote reservoir versions.

It's a shame that Kawasaki did not modify the cap so that it protrudes below the outside top edge of the rectangular remote reservoir, in order to hold it captive, but likely it took a good 10 years for these reservoirs to start bowing outward, so it was not an on the fly change during the middle of the KDX model production era.

Starting I believe in 2024, Kawasaki did change to the Yamaha / Honda (2004-ish+) reservoir shape, perhaps those would still have the correct half inch bore and fit better than the rectangular Kawasaki versions? I purchased one of the 2019-2023 Kawasaki versions which is just slightly modified from the earlier version 2003-2019 (different sight glass location), but it was not such a good swap candidate due to ill fitment with the stock aluminum master cylinder guard or the Works Connection versions.

[billie_morini]

Learned something new today. Didn't know about the reservoir deformation or its remedy. Will look at mine and add reflective heat shield to it.

[Chuck78]

So I've found that 2003-2004-2005 Yamaha YZ and 2003-2006 Yamaha WR rear master cylinders have the same spacing as the KDX and KX125/250 2-stroke rear master mounting pattern (I believe 49mm vs most modern integrated reservoir rear masters at 39mm hole spacing for aluminum frame Japanese bikes), and also have the banjo bolt line connection in a location that doesn't interfere with the KDX and KX 2-stroke frames aside from potentially needing to chop off the unneeded remote reservoir mounting bracket.
I have to credit this to Kale Meinershagen who is building a KDX240 big bore hybrid with a '99-'02 KX125 rolling chassis, he has this bolted up to his KX125 chassis hybrid now. He's the guy who is making and selling (on eBay) the vacuum-formed Kydex KDX clutch cover protectors and KDX and KX frame guards, among other protective cover type products for the Kawasaki 2-strokes.

These are however an 11mm piston/bore master cylinder, versus stock KX 2-stroke and KDX, which use 1/2" (12.7mm) pistons, so the hydraulic leverage ratio difference may perhaps make a very firm pedal that requires more effort.

The 2024+ KX450 and 2025+ KX250 rear masters mount up and clear the KDX and KX125 frames however, and have the same stock 1/2" bore piston size as the stock KDX and KX125/250 rear masters, but have the modern aluminum frame 39mm spacing of the mounting bolts, requiring the upper hole be redrilled. These look to be the best solution unless you have NLA older Works Connection frame guards which abandon the stock master cylinder guard mounting screws and are clamped on with the actual master cylinder mounting screws/bolts. The Works Connection guards have a recessed welded on bolt hole, so modifying these would be very involved...

I'll make a new post about this info with a few photos and part numbers.