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Rear Brake Occasionally Not Engaging

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I have an issue with my rear brake that I cannot figure out. When riding, the rear brake works 99% of the time. However, sometimes when riding I press the brake pedal and nothing happens. The pedal goes all the way down as if there's no brake fluid in the master cylinder. I then release the brake pedal and apply it again and the brake works fine. It's really weird because it seems like there's no brake fluid (or a bunch of air in the lines) when this happens. But I'm not sure that's the case considering the brake pedal works completely normal 99% of the time. It happens once or twice a ride and has caused a few close calls for going off the trail. The brake pads are brand new. Any input/ideas?

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For a quick fix I would try reverse bleeding as it sounds like there is air trapped somewhere. Bleeding the rear line is super easy due to how short it is.

For peace of mind, I would rebuild the master cylinder and caliper. You can get the rebuild kits for $17 a piece. How old is your brake line? If it's in good condition, re use it with fresh fluid after installing new seals in the cylinder and caliper. Then hope for the best.

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My guess is that the seal in the piston is damaged but still manages to seal 99% percent of the time.

I have seen this happening where the break would work if you applied pressure on the pedal vertically and it will not work if you applied pressure slightly to the side. It turned out that the seal was torn and when the pedal was pushing the piston a bit to the side the tear would not seal and oil would pass through.

Remove the piston and examine the seal carefully for tears.

 

 

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3 hours ago, jnk said:

My guess is that the seal in the piston is damaged but still manages to seal 99% percent of the time.

I have seen this happening where the break would work if you applied pressure on the pedal vertically and it will not work if you applied pressure slightly to the side. It turned out that the seal was torn and when the pedal was pushing the piston a bit to the side the tear would not seal and oil would pass through.

Remove the piston and examine the seal carefully for tears.

 

 

^^^^^^

This.

What's happening is the seal is caving in on one side occasionally. You can pull it apart and check the seal, but check the bore in the master too. If its got a knick in it somehow or the piston has worn it, it's junk.

You can get a rebuild kit from Honda, but I'd make sure it's rebuild-able first.

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You guys, and this forum, are awesome. Thanks for the help and I'll check it out as soon as I have time.

Edited by Thefischerman

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