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I bought a used ZX600E, my first motorcycle in over a year!

The bike is in really good condition, except it's been laid down in the front, and needed the fuel pump replaced.

I managed to re-align the front end, replace the fuel pump/filter assemply, change the oil, and tension the rear chain, however there is one problem that doesn't seem to go away no matter what I do.

The front brake (dual caliper system) does this unusual thing. You pump the brakes and get them as hard as you want, but as soon as you start moving, the brake pressure immediately goes away, and you have to pump the brakes again before the pads grip. the only way to alleviate this is to constantly apply a slight brake pressure.
Then I can feel a slight bumping of the brakes. My immediate suspicion was obviously there must be air in the brake lines, however after having bled them 3 times (note, only using the master cylinder, no special tools), and thoroughly washing out all the brake fluid, I am fairly confident there is no air in the brake lines.

I currently suspect the front rotors may be bent, however immediate visual inspection doesn't suggest that. It's a really weird phenomena that's not tied to wheel speed, but wheel rotation specifically.

Anyway, do y'all have any ideas what might be causing this?
The bike has been in a front end wreck, causing the front wheel to get pushed back to where it's hitting the lower fairing chin, as well as causing the handlebars to be slightly out of alignment, however I've corrected for that by loosening all the bolts in the fork system and straightening it myself.


Any help is really appreciated, this is the only thing keeping me from being able to ride!
 

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Your saying you bled the brakes 3 times only using the master cylinder.
I would say that you still have air in the lines.Start by bleeding the caliper
furthest away from your master cylinder. Next go to the other caliper
and bleed that caliper.You might have to do this a few times to get all
the air out of the system. That's assuming there are no leaks in your brake lines.
Just bleeding the brake from the master don't cut it unless your very lucky.
Getting a service manual would be very helpful.Hope this helps you out.
 

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Discussion Starter #3
Your saying you bled the brakes 3 times only using the master cylinder.
I would say that you still have air in the lines.Start by bleeding the caliper
furthest away from your master cylinder. Next go to the other caliper
and bleed that caliper.You might have to do this a few times to get all
the air out of the system. That's assuming there are no leaks in your brake lines.
Just bleeding the brake from the master don't cut it unless your very lucky.
Getting a service manual would be very helpful.Hope this helps you out.
What I meant to say was I didn't use a pressurized system to pump the brake fluid out, I used the master cylinder (brake lever) to push the fluid out of the system.
I even removed the brake calipers once and used tie straps to raise them (pushing all potential air upwards), then bled using the brake lever, again, no air was found.
 

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Might be your m/c is no good.When you hit the brake lever and pump it up the brakes do grab.
so most likely your calipers are good.When I bled my brakes I use the brake lever.I really don't
think using a pressurized system to bleed the brakes would make any difference.I sure one of
the other members here have some useful information.
 

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How are your brake lines? I'd also check that, when they get porous your brakes get
that spongy feel to it.One thing for sure get those brakes fixed before you go out on
the road.Keeping some pressure on the brake lever might work for now they may also
crap out when you really need them.It's not worth risking your life for.
 

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The disc brake rotor is bent. It's pushing the caliper piston back a little bit each revolution. You won't be able to see the problem with the rotor, you need to measure the runout with a dial indicator. If there was a problem with the master cylinder or air in the system, you wouldn't need to move to have the problem occur.
 

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Hi. Were you able to resolve your issue. I'm have a simular problem. On my bike the wheel--( life off the ground for repairs) will become frozzen for any rotation after siting over night.The brake handle will become very hard and the pads are hard up against the rotor. 2005 800 vulcan kawaski. change the pads for new and replaced the rotor and wheelbearings.
 

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Fried Chicken
A poor mans dial caliper is a screw driver set on top of a box with the tip just touching the disc. Rotate it slowly and see if it stays in contact all the way around. You can use feeler gauges to measure any clearance. The disc will push the screwdriver back when it is bent towards it ans get a gap when bent away. It will not take much of a bend to get the condition you describe.
willp
You have the opposite problem. Your pistons are not pulling back off the disc when you release the breaks. If you changed the pads and didn't rebuild the calipers or even clean them very well, you will have pushed dirt back in alongside the piston when you installed the pads and that will cause the pistons to bind in the extended position. If they are only sticking a little bit you might not notice it when you are riding. The bike puts more force on the wheel than you can by hand.
 
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