98 YZ400 Rear Caliper Troubles

8 replies to this topic
  • motocrosschump

Posted August 19, 2004 - 06:34 PM


I replaced the brake pads on my bros 400 and the piston was extremely hard to push in. When I put it back on the bike and when I hit the brakes, the brakes drag really bad. Is this something I can fix? Or Do I need a new caliper?

  • Matt96xr6

Posted August 19, 2004 - 08:36 PM


you need to rebuild the caliper with new seals and then hone the caliper slighlty and rub the piston with a bit of emery cloth.

This is if there is no damage to any parts.

  • Big_D

Posted August 19, 2004 - 09:43 PM


Matt is dead on!

Instead of using emery cloth on the piston I would use the finest grade of scotch brite pad you can find with a little brake cleaner to make sure that there are no deposits on the piston. Polish that thing up. They usually get junked up when they are exposed for a long time because the brake pad is worn thin.

  • Hamish

Posted August 20, 2004 - 02:12 AM


Matt is right. I'll also add that when changing pads you should never just push the pistons back in. You need to pump the piston out slowly (not too far!) so you can clean it, otherwise when you push it back in the dirt that was on the piston will cause the problem you are having

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  • motocrosschump

Posted August 20, 2004 - 04:58 AM


Does yamaha sell replacement pistons? That thing is brittle.

  • Hamish

Posted August 20, 2004 - 06:00 AM


yep. see www.yamaha-motor.com for the p/n


Posted August 20, 2004 - 07:54 AM


I've seen this many times before.... Also take a look at the brake pin..... If you have not changed it in a long time, it will wear a grove that the inside pad gets hung on and will cause the brake not to release. additionally if the brake gets compressed past the groove the brake will not return and the the pedal will get hard.

Pins are cheap.

Bonzai :thumbsup:

  • SWD6

Posted August 20, 2004 - 07:59 PM


Check for corrosion in the caliper piston seal grooves.Moisture in the break fluid I think causes this and just replacing the seals without cleaning the grooves won't fix the problem.

  • rsstom

Posted August 21, 2004 - 03:57 AM


And make sure your fluid level is correct. As the pads wear, you've topped it off. Then you replace the pads and there's now too much fluid in the reservoir.
Sometimes it comes squirting out when you push the piston back in.
Sometimes it doesn't and does what you're describing: Hard to push the piston back in and the fluid is now putting pressure on the piston, causing the brake to drag.

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