rear brakes

i have an 06 300 xcw,when i come down a long hill where i have to ride the rear brakes,it heats up so much the rear tire locks up solid,i've rebuilt the rear master cylinder and rear caliper and it's still locking up,any suggestions

Make sure the piston in the master cylinder is returning all the way to the rest position.  Also make sure there is some slack in the rod that pushes against the master cylinder piston.  If these 2 items are not correct, it will keep the fluid retrun hole in the master cylinder bore covered.  As the fluid heats it can't return to the reservoir.

 

Also, even though you rebuilt your caliper, make sure the caliper slides freely on the 2 slide pins.

 

Not sure if any part of your rear brake line is rubber, but if so, the inner liner of the rubber hose can collapse, and it can act like a check valve.  Although it can do it anytime, it can worsen with heat

This will be no help for your mechanical issues, but...  As a technique, Try not to use the rear brake on downhills.  The front brake does most of the braking on the flats and even more so on the downhills.  Also you have much better directional control when the rear wheel is turning.  So let the rear wheel roll and use the front brake as much as possible. 

This will be no help for your mechanical issues, but...  As a technique, Try not to use the rear brake on downhills.  The front brake does most of the braking on the flats and even more so on the downhills.  Also you have much better directional control when the rear wheel is turning.  So let the rear wheel roll and use the front brake as much as possible. 

And then I watch some HARD Enduros on YouTube and see some of the worlds best riders doing steep down hills with the rear wheel locked up, ..... So never mind.  If my technique is not good enough for them, it's not good enough for you.  My Bad

I personally alternate between front and rear If I'm doing a long technical downhill section. I  have boiled my fluid once on an incredibly long downhill portion in AF canyon. It sucks. fortunately there were some large trees to stop me as I missed the turn because I had no rear brake anymore. Since then I consiously alternate between front and rear let one cool down while the other gets hot.

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