you have a leak somewhere in the rear brakes since air is getting in somehow. I could make sure all the lines are tight that go to the master cylinder, the caliper, and the bleeder screw on the caliper. you could also has a fail in the master cylinder. or a pin hole in the line. Also, take the reservoir cap off and make sure that rubber boot thing is fitting right or is even the correct boot for your bike. If none, of that works, I don't know what your problem is.