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Discussion Starter · #1 · (Edited)
Background info:

I've got a serious problem on my newly acquired K636 which I'm planning to use mainly as a track bike. This bike was originally a 2004 model that got completely submerged while being stored in New Orleans during Katrina. During the rebuild the previous owner installed a 2006 front end which uses a Nissin radial brake master cylinder.

When I got the bike the orientation of the brake lever was up higher by about 15-20 degrees than the clutch lever. That drove me nuts -having different angles on the two levers plus the brake lever was up higher than what I deemed comfortable. Rotating the brake lever down was not possible due to clearance issue with the clipon. I'm not sure if the clipons were 2006 or the original 2004. Anyway after I found my Dremel tool there were no more clearance issues and I rotated the brake lever down so it matched the clutch lever and felt comfortable to me. The only other changes I made were installing steel braided brake lines and I fashioned a little aluminum mount for the brake reservoir. The reservoir was ziptied to the top triple clamp when I got the bike.

Problem:

The friggin brakes slowly come on by themselves. I'm at the point now of taking a 8 and 10 mm box wrench with me when riding so I can crack open the bleed screws and relieve the pressure. I've done this at both calipers a couple times and today I tried it at the MC bleed screw which is easier to get to and there's only one so its quicker two. Both approaches work. After venting the system I'm good for a couple hours of riding. I've never ridden much longer than that so i don't know if I would have to vent again on the same day. Then the next time I take the bike out on a different day same thing. Within the first 20 minutes or so of riding I notice the pressure go up. I vent it and I'm good.

My first thought was I had a pad hanging up on the rotor heating the fluid an making it expand thus increasing the pressure. I removed the pads and cleaned all the pistons real good and now I'm sure that's not the issue. When I stopped to vent the system the last few times the rotor and calipers were not hot at all. I think the problem is related to thermal expansion of the brake fluid. Its cooler in my garage than outside. When I take the bike out the fluid heats up due to higher ambient temps and expands increasing the pressure in the system.

The first time it happened when I didn't know the issue existed I was on the freeway and the brakes came on strong. I felt the resistance to forward motion and thought the motor had seized. I pulled in the clutch to coast off the highway and the bike came to a quick stop right on the edge of the lane and then I couldn't push it on to the shoulder cuz the front wheel was locked.

Any ideas on this would be most welcome. I can't imagine tilting the MC forward would cause the problem. After all when you hit a downgrade on a mountain road the lever will get tilted forward and it still must work. It is likely I have a busted brake MC?

EDIT: one more thing, after cleaning all the pistons I bled the system using a new bottle of valvoline synthetic dot 3 & 4 compatible brake fluid. So the problem shouldn't be fluid related. And I have some experience with brakes. On my SV1000S I split the calipers to powder coat them and rebuilt with all new seals. I put on steel lines and replaced pads a couple times. So I know how to bleed the brake syetem. Although this is my first time with a radial brake MC.
 

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You may want to change it to a striaght DOT4 brake fluid. Last time I bled the brakes I went to through an entire bottle for the fronts when bleeding and a half bottle for the rear. I know the 03/04s called for DOT4 only. If you use DOT5 it could negatively effect your seals on the 03/04s but I don't know what the 05/06 calls for.
 

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i experienced a very similar issue to yours. after a tire change my front brake wouldn't hold much pressure. after research i adjusted the brake lever and the problem changed. the brake would gradually bite during rides. i found the problem to be inside the master cylinder. after i replaced it the brake worked fine.

from what i've found after research and experience, the front master cylinder and it's components can be very sensitive to adjustment of any kind.
 

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You may want to change it to a striaght DOT4 brake fluid. Last time I bled the brakes I went to through an entire bottle for the fronts when bleeding and a half bottle for the rear. I know the 03/04s called for DOT4 only. If you use DOT5 it could negatively effect your seals on the 03/04s but I don't know what the 05/06 calls for.

05/06 636 calls for Dot4 brake fluid
 

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DOT4, Stay away from dot5. +1 on replace the front Master Cylinder
 

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Discussion Starter · #8 ·
Went for a ride Thursday and when I noticed the pressure building up in the system I removed the lever instead of venting the hydraulics. The pressure was immediately relieved. So I put the broken stock lever back on and all seemed fine (except now no brake lights). Tested again today with the stocker and rode for 2 hrs with no issues. Put the replacement lever back on just to verify the brake light switch - brake lever clearance isn't related to the problem and it appears its not. So Problem solved. The cheap replacement Hong Kong lever from eBay must not allow the MC piston to return all the way to the home position. OEM lever ordered.

You peeps might want to keep this experience in mind if you ever damage your stock brake lever and are thinkin about scoring a $15 replacement on eBay.
 

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Glad you figured the issue out, good work. Now instead of stockers, run Pazzos or ASV levers. Feel better, look better, are available as shorties and just plain rock. Not to mention they are way more adjustable, including the clutch. I've had Pazzos on my bike for a long while and will never go back to stock. One of the fist things I will change on a new bike after doing brake lines.
 

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Sounds like the same damn thing that happened to me a little while ago. I was about 1.5 miles away from home when I went to upshift doing about 40 mph and suddenly might bike starts to jerk and progressively slow. I pulled the clutch in right away and started braking and managed to get into the turning lane in the middle of the road. Not much traffic was around so I'm glad of that. At first I thought it was my transmission or something that was messed up but I tried to roll the bike in neutral and the front wheel wouldn't budge. After a couple minutes of all kinds of stuff going through my head I noticed the brake lever adjustment dial was not where I keep it. I turned it to adjust it and immediately the brakes released. I pulled off to a side street and checked the brakes. The lever felt like it was still too close so I adjusted it a little more and rode about 10 miles or so at relatively slow speed while stopping many times and the brakes felt fine.

Anyone know if the brakes may lock up like that if the lever adjustment dial is maybe in between two settings?
 
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