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Changed Front Pads on 954RR. Now the lever is pulsating.

Discussion in 'Technical and Troubleshooting Torque' at netrider.net.au started by jaguarfanster, May 14, 2013.

  1. I recently changed the front brake pads on my 954rr. Firstly I noticed the lever became a little 'squishy' after the pistons were pushed back to fit the new pads. I don't know how air would have entered the system given I didn't see any leaks. Anyway I understand I'll have to bleed the brakes? My procedure will start with cracking the banjo bolt to bleed the master cylinder and then onto bleeding at the calipers. Please let me know if that's right.

    Anyway on a test ride I noticed that the brake lever pulsates. This never happened before the pad change so I highly doubt the rotors are warped. Could this pulsing or 'grabbing and slipping' be associated with air in the lines, as above, or another issue?

    Thanks.


     
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  2. A spongy lever after fitting new pads doesn't necessarily mean that you have air in the system requiring bleeding. I have seen this many times after fitting new pads. sometimes it is due to the discs having a little wear & because the pad surface has not worn in to the same shape as the disc yet they are a little 'springy' and will improve once the pads have bedded in. Sometimes it is due to the caliper pistons having been disturbed & the orings are pulling them back away from the pad further than before, in which case a zip tie on the lever overnight might help with this as well as dislodging any micro air bubles in the system.

    Did you clean the outside of the caliper pistons before pushing them back into the caliper?

    Did you have any difficulty getting the calipers off the discs requiring you to tilt the caliper on the disc or lever it in any way?

    Pulsating brake lever usually means bent or worn disc, however badly worn wheel bearings can also cause this.
     
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  3. As for what Tinkerer has said.

    I would bleed brakes again (or replace the brake fluid ensuring you use the correct fluid for your bike; and buy new brake fluid to use, old stuff can absorb water very easily), leaving lever compressed overnight, tapping the lines to release any small bubbles that my be caught in the lines.
    New pads need to be scrubbed in, some stick a wire brush or sand paper over them; however, I just ride the bike and accept that it will take a day or two to seat in properly.
    Pulsating usually indicates warped / worn disc.
    If you pull in the lever and hold it; does it slowly still compress further?
    If so, this may indicate air in system or your brake lines may be bulging (doubtfull though if you have braided lines) or has a leak.
     
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    • Agree Agree x 1
  4. Cheers for the input. Anyhows I've curbed about 95% problem after taking a scotch brite and rubbing off some rust I spotted on the rotors. After that I removed the pads and with a 120grit sand paper sanded off some rust/gunk on the pads. Stops a lot better now. Although I suspect the rotors may be ever so slightly warped.

    The cable tie on brake lever method is a novel to me. I did a bit of reading and most recommend tying the brake lever for a night, releasing the lever slowly next morning, and allowing the brake to sit for a half day before usage. Something about letting the bubble work it's way into the reservoir...
     
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  5. The compressed bubble theory doesn't wash with me completely.

    Any larger bubble will be able to rise through a liquid more easily than a small one, unless it is the same size or nearly the same size as the hose it is rising through, which would constrict the space available for the fluid to flow down and around it (in relative terms) as the bubble rises. Compressing the bubbles might work where the bubbles are the same size as the lines or nearly so. That's big. The logic is sound where there the bubbles are big.

    Where the bubbles are much smaller, compressing them would actually impair their rise, due to increasing their density. Very tiny bubbles will possibly not rise at all due to the viscosity of the fluid.

    Where I've had difficult bleeds, I've had the most success with bleeding them backwards, from the bottom, using a syringe and hose attached to the bleed nipple. Just ensure that you gently suck the air from the hose into the syringe first, then wait, allowing the bubbles to rise to the top of the syringe before squeezing fluid into the calliper. You can get a suitable syringe at many autoparts stores - like Supercheap. Often sold to measure precise amounts of oil The one I've got holds 80 ml. That's plenty big.

    NB, keep an eye on the level of the fluid in the reservoir, you don't want to pull air in from the top, and watch to see the bubbles in the line arrive in the reservoir, as you push fluid up the lines, be careful not to overfill the reservoir (paint is affected by brake fluid). I generally work the fluid back and forth using the syringe and the lever. The technique is the best I've found to eliminate air bubbles. I'm too old to wait for the combined effect of bouyancy and gravity to do their things.

    NB2, If you suck too enthusiastically to empty the hose of air into the syringe, when you start, you may draw air in through the threads of the bleed nipple. This is normal. If the threads become a little wet with brake fluid, it provides a better seal. Don't loosen more than you need.
     
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