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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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.
It's almost a bit normal
Your brake pads aren't lined up on the wear grooves of your disc.
What is very common on dirtbikes is to get a few grooves, not necessarily deep, cut into the discs/rotors. When you put the wheel back onto the bike the pads on your front caliper were just a little bit off.
There really isn't any tool or special alignment you can do to remedy the issue. It's almost a "well, this is the way it is" thing.
What you can do is get your pads retracted back just a little bit so you can move the brake pad up and down just a bit. You try and feel where the pad best hits the disc and see if you can feel the pad fitting into the grooves. When you feel it fit in, you know it. Once your there, pump your front brake lever a couple times to get your cylinder pushing out against the pad.
If you can't find the 'spot' then you'll have to ride the bike and get the pads worn back into the disc.
This isn't bleeding or anything like that. You took your wheel off. When you put it back on the brakes are lame. If you overthink it your will end up doing a brake job to resolve something you won't resolve by doing all that activity. Try it the next time you swap a tire or tube. It usually lines up and everything is fine.
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.