Brakes fine 2,10,30,...NO TELLIN'? miles of riding on/off road ...Brake ( LOCKS ) ?? Pull over bleed screw to spread discs ---works fine then it happens again.PS A Complete new rear brake assembly was already installed.With the Same result??
Check the master cylinder. When this happened to a buddy of mine he found a piece of crud blocking the return.
With the info you have given, I would suggest checking/replacing the master cylinder.
Do as Dani G and Firecracker say. Also check that the rubber hose is not collapsed on the inside or the internal check valve stuck. If any of these are found, you will need to replace the hose. If any of this happens, it will not allow the fluid to return.
The most probabe cause is a lack of lubrication and/or corrosion in the linkage from the pedal to the brake shoes, or in the shoes themselves. Easy to tell: With the bike on the sidestand, manually push the brake pedal and observe the linkage. If it appears stiff, take it off and see if it moves freely. My guess would be that you will have to take the rear wheel off and see what your brake shoes look like. May just need to carefully lube the springs and pivot points. One thing in your favor is that this is one of the simplest motorcycle braking systems around, and the rear wheel comes off easily. Chilton or Haynes manual is only $10-$15.
It certainly could be the master cylinder, but even though you state that a completely new rear brake assembly was installed, it still sounds like you have a binding brake caliper. You may have to disassemble it before reassembling with plenty of anti-seize compound.
