Rear disc brake is locking

[baileyniner]

After riding the bike for 5 minutes the rear brake system seems to go under constant pressure and locks the pads down on the rotor. I can hardly push the bike around in neutral. I bleed the brakes but no change. i heard there is a small relief port somewhere on the master cylinder or plastic reservoir that may be clogged. Is the port on mc or reservoir. Once cooled down the brakes return to normal until riden again

 

[sgallaty]

this sounds like a relief valve. you're going to have to rebuild the MC to fix this problem. Also check for binding in the floating caliper slide which is a greased fitting under that rubber boot in the bracket.

 

[Tomterrific]

At the top of the bore of the MC is always two small holes. The smallest is a pressure relief. This smaller tiny hole sits right in front of the rubber lip seal of the MC piston. The seal goes past the hole when brakes are applied and when teh brakes are releaced the hole releaves any pressure excess build up from heat. The larger hole is a fluid hole to the reservoir.

A speck of dirt can get into the tiniest hole and the pressure holds it there so the excess fluid can't escape. The pads start to drag and the caliper heats which makes more pressure, more drag, more heat and so on until the brake locks and 'CRASH'. A fix can be as simple as a poke through with a thin wire and a good system flush. No matter if the entire system is super clean, that tiny hole needs to be clear. A good poke with a thin wire is as good as anything to get it open.

Tom Graham

 

[sgallaty]

At the top of the bore of the MC is always two small holes. The smallest is a pressure relief. This smaller tiny hole sits right in front of the rubber lip seal of the MC piston. The seal goes past the hole when brakes are applied and when teh brakes are releaced the hole releaves any pressure excess build up from heat. The larger hole is a fluid hole to the reservoir.

A speck of dirt can get into the tiniest hole and the pressure holds it there so the excess fluid can't escape. The pads start to drag and the caliper heats which makes more pressure, more drag, more heat and so on until the brake locks and 'CRASH'. A fix can be as simple as a poke through with a thin wire and a good system flush. No matter if the entire system is super clean, that tiny hole needs to be clear. A good poke with a thin wire is as good as anything to get it open.

Tom Graham

True tom, but I'm suspecting that he's got binding due to relief valve blocking and themal runaway on the hot caliper as you're suggesting. (He could have several problems, naturally). A binding floating shaft could also cause drag that will heat it up.

Whenever I've seen the intake and relief holes blocked though, it's usually been crystallized or gelatinized brake fluid. Brake failure is dangerous enough that I suggested he tear it down, clean it up and be done with it.