I have a Yamaha 350 Bruin 2WD atv. The front brake calipers are not releasing and will lock up by their selves sometimes when just sitting, not running. The only way to get them to release is loosening the brake line at the reservoir and bleeding a little pressure. They immediately release but will soon lock up again. I have flushed and changed the brake fluid, disassembled and cleaned the calipers and installed new o rings on the pistons. I worked for two days to make sure all the air was bled out of the system. After all that they worked OK for about a week and it started all over again. Anyone have any ideas?

The brake hoses from the chassis/frame to the caliper/wheel cylinder is your problem most likely...

By your description, it sounds like there's an issue in the master cylinder. If you can crack a line at the reservoir (I assume it's mated to the master cylinder) and everything releases, then something in the master cylinder isn't releasing or holding pressure when it shouldn't. You can try disassembling it, cleaning everything with clean brake fluid, clearing the small orifices, etc, but most likely it'll need a rebuild kit at minimum.

Hope this is helpful.

Don't understand what problem you are talking about. The hoses are not kinked or cracked and do not leak. They were flushed out when I did the calipers. Am I missing something?



Forgot to mention in the original post I installed a rebuild kit in the master cylinder as well when I did the calipers. Looked like it was going to be the answer but went back to the same thing after about a week. By the way, when they started locking up, I hadn't started or ridden the atv. It had just sat in the shop and every day I would go by and rock the atv back and forth to make sure it was still rolling. After about the 7th or 8th day, they had locked up again.

The rubber hoses collapse internally over time (sometimes) and will only let the brake fluid pressurize not release...

I had a similar problem. I found that my ATV brakes were on partially all the time. You could drive it, but you couldn't push it. I was on the verge of taking the whole brake system apart when I found that the foot brake (mechanical, rather than hydraulic) had rusted in the down position. Much penetrating oil and working the brake arm up and down, and it was Ok.

Haven't checked the inside of the brake hoses. Haven't thought about the foot brake either. Thanks for the suggestions guys.