My brakes have always been a little week but now my rear brake is not working at all. I have tried to bled it, but it is not even getting spongy, there is no resistance when you step on the lever. Has anyone else had this problem? do I need a new master cylinder?

 

If you are not loosing fluid, I would suspect the master cylinder. There are 2 rear calipers on the same rotor, one driven by the foot pedal and one by the hand brake lever. So if the foot brake isn't working, you still have a rear brake.