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I have a 2008 Kawasaki ER6 / Ninja 650R race bike. Within my class I must use OEM calipers & a OEM master cylinder. These components are also not allowed to be modified.

As the title says, the brake lever has a very soft feel and the lever's travel from zero braking to maximum braking is very long. The brake fluid gets changed regularly and bled often. The calipers and the master cylinder have been recently rebuilt by a qualified technician and the bike has braided hoses.

None of these things have made much of a difference. The braided hoses have been on the bikes since I bought it, but as far as I can tell, they are in perfectly good condition.

What else can I do / check to try and improve this?

Thanks in advance
Willem

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  • Something definitely not adding up. Have you measured rotor run out? I would hope to see 0.010” or so in the middle of the rotor. A large run out would require a whole lot of travel.
    – zipzit
    Commented Feb 21, 2019 at 19:49
  • I must be honest, I don't really know. How would I check this? The discs were also replaced if that helps?
    – Willem
    Commented Feb 21, 2019 at 19:55
  • Use a dial indicator with an adjustable holder. Here’s a photo of a car rotor. image.superchevy.com/f/175473287+w640+h640+q80+re0+cr1+st0/…
    – zipzit
    Commented Feb 21, 2019 at 23:05
  • While rotor runout would affect brake operation when the bike is moving (you'd feel vibration in the brake lever, and the grip amount may vary as the wheel turns), too much run out will have nothing to do with how the lever feels when the bike is standing still.
    – DavidRecallsMonica
    Commented Feb 22, 2019 at 0:36
  • David, fair points.. but. 1) The original question made no mention that the "bike is standing still" Not one word. 2) He hints that he understands master/slave/brake lines expansion/ bleeding. 3) He specifically asked "What ELSE can I do...?" I've seen rotor/ axle/ runout related issues cause excessive slave cylinder travel withOUT accompanying vibration, albeit in automobiles, not a bike. My suspicion is something is pushing the wheel pistons far apart, causing the lever's travel from zero braking to maximum braking is very long, perhaps a wheel bearing, or a mount alignment issue.
    – zipzit
    Commented Feb 22, 2019 at 5:16

2 Answers 2

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The cause of a soft brake lever is one or more of (a) air in the brake lines, (b) tired brake lines that expand as brake fluid pressure increases when the brake lever is pulled, (c) failing master cylinder, (d) failing wheel cylinder(s), and/or (e) a physical leak in the brake system that yields a puddle on the garage floor and may allow entry of air, causing (a).

(c) and (d) failures may not be evident externally. If the brake fluid level in the master cylinder reservoir isn't going down over time, then the problem is not (e).

You will need to examine the brake system, and rectify any fault(s) you find. Only because it's a very common problem, I'll guess the issue is (a). Find good instructions on how to bleed your machine's brakes, and follow them. You might ask other Ninja pilots how they do the job.

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  • Hi David, (a) The brakes are bled regularly and not just by myself (b) The brake lines are braided hoses, could they be past it or of an incorrect spec/diameter? There are no visible leaks. (c) the master cylinder was recently rebuilt with new OEM internals, could the master cylinder be so past it that a rebuild would not be sufficient? (d) The calipers were also recently rebuilt, I'm assuming wheel cylinder = calipers. (e) There are no visible leaks and no puddles near the bike.
    – Willem
    Commented Feb 27, 2019 at 18:06
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    If you cannot obtain a hard lever after bleeding, then either you still have air in the system (perhaps trapped at one or more of the wheel calipers), or one or more of the brake lines are expanding under pressure, or the master cylinder piston seal has failed, or the master cylinder bore isn't sealing to the master cylinder piston rubber seal. Do you get clean no-bubble fluid at each caliper when you bleed? I've never heard of braided lines failing, but anything's possible. If the fault is at the master cylinder, I'd replace the entire master cylinder with new.
    – DavidRecallsMonica
    Commented Feb 27, 2019 at 18:29
  • No bubbles when bleeding the brakes, I've bled them myself and two mechanics have bled them on separate occasions. I'm considering replacing the master cylinder and changing it to a newer model, perhaps it will give better results. Do you think I should replace the brake lines as well?
    – Willem
    Commented Feb 27, 2019 at 19:39
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    I've never heard of a braided line failing, and have heard many times about master cylinder failure. Thus, I think the fault is more likely to be in the master cylinder. I'd replace it with new, then fill and bleed again.
    – DavidRecallsMonica
    Commented Feb 27, 2019 at 21:37
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do you have ABS? if not, it's air, or a damaged worn master cylinder (the pump on the handlebar)

if you have ABS it could be the abs module

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