Rear Brake Lockup on KX250

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Rear Brake Lockup on KX250

Postby mikezolin » 10:22 am Jul 10 2006

The KDX is having the topend redone so I took my son's KX250 to the track yesterday. Practice was split with the nov, int, pros in one group for 20 mins then us beginners and little bikes for 20 min. First session no was fine - riding slow and just having fun. About 10 mins into the 2nd session accelleration began to suffer and when I let off the throttle the bike slowed very rapidly.

Hobbled back to the truck and the rear brake was under pressure. The heat coming off the rotor was intense. I popped the bleeder valve and it released the pressure. No issues since.

What the heck would cause something like that to happen? The brake lever was not stuck??????????????
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Postby skipro3 » 10:50 am Jul 10 2006

The caliper probably jammed in it's cylinder.
The way the hydraulic disc brake works is that a puck is pushed out of a cylinder under pressure from the lever being pressed and moving brake fluid. If the puck doesn't slide smoothly in it's cylinder, it could get a little sideways and jamb. It probably jambed up under pressure. This usually happens when new pads are installed and the puck is shoved back up into it's cylinder to make way for the new thickness of a fresh brake pad. If there is any moisture in the brake fluid, and it found it's way to the cylinder behind the puck, it would cause corrosion. Then when the puck slides over that part of the cylinder, it could stick. This is good reason to bleed your brakes regularly. The brake should be disassembled and inspected for contaminates in it's cylinder. They can be honed and rebuilt if there is any corrosion.
Hope this helps.
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Postby canyncarvr » 12:17 pm Jul 10 2006

IF the piston was stuck, that wouldn't account for the pressure?? Unscrewing the bleeder in that case would not result in pressurized fluid coming out.

Well...the question is then, what happened when the bleeder was 'popped'.

Is the level in the MC correct?

Is the rotor true?

Pedal adjusted?

Brake pins can cause a pad to hang up. Usually that results in crooked wear and lousy pad life..NOT a lot of pressure and heat.

Could be junk in the system, too. Bleed it (use suction). Then back-bleed it (pressure from the slave TO the MC).

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