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After changing the air tube in my front tire and reinstalling the wheel on my Suzuki RMZ450, I noticed that the front brake lost a lot of stopping power. What caused this and what do I need to fix to get the full stopping power back? Do I need to bleed the front brake? Please give step by step instructions if possible.

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What year is your bike? –  Seminecis Sep 5 '13 at 21:03
    
@Seminecis - It's a 2007. –  hw731 Sep 6 '13 at 13:02
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2 Answers

up vote 4 down vote accepted

It is unlikely that simply removing and re installing the wheel would cause air to enter the brake system, though I suppose if someone had repeatedly squeezed the brake lever while the wheel was off the bike, that could have run the master cylinder reservoir dry and admitted air. One should never apply the brakes unless brake pads and brake rotor are all present and in place.

If air was in the brake system, the lever should feel "mushy" compared to before. If it feels mushy, go ahead and bleed the brakes.

I would also check to make sure that the pads in their proper position in the caliper, and make good contact with the rotor. It's possible they got knocked around during the wheel change.

It is also possible that the pads or rotor have simply become contaminated during the wheel/tire change. If the lever feels firm and you have decreased stopping power, this may be the cause. Try spraying everything with some brake cleaner.

As to how to bleed the brakes, you may need two people, though on a motorcycle you can generally do it with only one.

  1. locate the bleed nipple on the brake caliper, and ensure that you can loosen it. If it is stuck, hit the threads with penetrating oil and use a flare nut wrench or a six-point socket to break it loose. Gently re-tighten the bleed nipple.
  2. locate the brake fluid reservoir on the master cylinder, and remove the cover. protect any paint in the area, as brake fluid can quickly damage paint. Also, keep this stuff off your skin. Check that there is some brake fluid left in the reservoir, as the bleed process will consume brake fluid and you must not run the reservoir dry or you will pull more air into the brake system, and this is the very problem we're trying to fix.
  3. affix a piece of clear tubing to the bleed nipple, and run the other end of the clear tubing into a small plastic bottle or other suitable container to hold the old brake fluid.
  4. with the bleed nipple closed, squeeze the brake lever (or press the brake pedal). While keeping pressure on the lever (from here out, substitute "pedal" for "lever" if appropriate), loosen the bleed nipple until brake fluid flows into the tubing.
  5. When the brake lever reaches the end of its travel, hold the brake lever in place while gently tightening the bleed nipple.
  6. With the bleed nipple closed, release the brake lever.
  7. Check that fluid remains in the master cylinder reservoir, and if required, top up with new brake fluid from a new, freshly-opened container of brake fluid. Again, the reservoir must not be allowed to run dry during the bleed process.
  8. Repeat steps 4-7 until no air bubbles are visible in the fluid exiting the bleed nipple. Note: if you wish to do a complete brake fluid change (you might as well), continue to bleed until the fluid coming out of the bleed nipple is light in color and clear, like the new fluid you're adding.
  9. When finished, snug the bleed nipple down, replace the dust cap (if present), top up the master cylinder, and replace the master cylinder cover. Clean up any brake fluid spills quickly with lots of soapy water.
  10. Test for brake feel before riding/driving the vehicle. The lever should be nice and firm.
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Wow thanks for the detail! I'll try this out today. –  hw731 Sep 6 '13 at 20:04
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I agree with @mac it shouldn't happen after a simple tire change. Have you tried bleeding the brakes? There could be some air in the fluid that is loosing "breaking power" when you apply the brakes. To do this there is usually a nipple on the caliper that you can loosen off slightly then press the break leaver softly and slowly till some fluid comes out. While is it still coming out close off the nipple and it should be fine. After this you should always clean your disk to make sure no fluid got on it.

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