Rear brake lock up while riding

[bonesaw]

A friend of mine was saying his rear brake engages while riding without pressing the pedal.he has disc brakes and I've never heard of anything like this before... Any insight or problems you guys can think of?

Thanks

 

[gggGary]

Yup it's happened to me, Most likely the master cylinder needs an overhaul, the return port is blocked so the brake fluid in the system is "sealed". The brake drags, warms the fluid, it expands and increases the pressure on the pads, heating the fluid further......... It can quickly ramp up to a fully locked brake. Stuck pads/caliper pistons will drag but they are not as likely to fully lock up the wheel. Always a good practice to R&R all the brake components on 30+ year old motorcycles.

 

[bonesaw]

Thank for a solution and teaching me something.thanks gary

 

[zorpman]

Old Japanese disk brakes tend to drag on the rotor. This creates heat. If there is ANY air or water in the brake fluid it expands with the heat and the problem increases exponentially as the fluid pressure is applied to the pads through the calipers. Brake drag increases and can lead to a fully locked brake. I would bet that when the brake is locked that opening the caliper bleed nipple will result in a hard squirt of extra hot brake fluid/water vapor.

Change the brake fluid with new Dot 4 (as in a fresh bottle of fluid with the seal still intact) and correctly bleed the air from the system and you will probably solve the problem. It's also a good idea to carry a nipple wrench with you after you mess with the brakes. You never know when you might have to unlock your disk the old fashion way.

 

[bonesaw]

Thanks Z I appreciate it,I'll relay the info

 

[XSLeo]

New fluid won't hurt. It should be changed out every couple years anyway. Regular changes help keep things clean.
The master cylinder has two holes in the reservoir. One feeds fluid to the piston. This is the larger hole. The other hole is smaller and is a pressure relief. If this small hole is plugged, As zorpman said as the brakes drag, heat builds up this heat expands the fluid, air, water, ect. If the pressure relief hole is plugged this fluid expansion presses the pads on the rotor and can lock things up. If this pressure relief hole is plugged, then changing just the fluid might not fix the problem.
I would take the M/C off the bike and Do a tear down, cleaning and inspection. If all the rubber parts are good just reassemble and reinstall.
Might even do the caliper. Won't hurt.
Leo

 

[5twins]

Yes, being so old, these bikes usually require both the MC and caliper torn down and cleaned. You will be amazed at the goop you find in there, lol.