You probably need to rebuild the rear caliper.
Here's what I did to my machine.
Take the rear caliper off and disassemle it to get to the brake piston. (The sliding piston has one of brake pads on it.)
The piston should slide freely inside the caliper, but it is probably corroded and filled with dirt. Clean the piston and make sure it doesn't have any big scratches that will keep it from sliding smoothly. Grease it up and put the caliper back together.
Another thing you may want to do is replace the rear brake cable. The stock cable is poorly designed because it goes up to the handlebars for your parking brake. When the cable starts to stick, the brakes work poorly. I bought a cable off eBay that eliminates the portion going to the bars. It works much nicer and came with the pins and adapters required to install it.
The rear brake on my machine works great now. Understand that a mechanical brake will never work as easily as hydraulic, but I can lock the rear tires.