Did they work before you changed prop valves? If so, put the stock one back, and just cut off the arm.
The rear brakes are self adjusting. But they are usually sticky, and don't adjust on there own very well. Jack up the back, on the backing plate is a rubber rectangle plug with rounded ends. Remove it. Using a flat head screw driver you should be able to reach inside and rotate the adjuster knob. I can't remember which way to turn it though. It's either up or down. It has teeth, if you try one direction and it feals like it's slipping over the teeth, go the other way. You'll be able to feal, and hear it click. Count how many clicks you do on each side. Adjust them til there is just a little bit of drag when you try and spin the tire.
Hope this makes sense.