I'm running rear disks. Larger bore master cylinder and proper bleeding cured my ills. You need to make sure the bleeder valve is higher then the piston. If the bleeder valve is lower then the piston on the caliper you will have air bubbles.
After I got mine working properly, I had more braking in the rear then I wanted. (locking up) I tossed the actuating arm for the brake bias thingy on the frame over the rear axle and set the rear brake pressure to a comfortable setting by trial and error. Then locked it there. you can use baling wire.
