I ride an 83 Lowrider and the rear brakes went out on the way to work the other day! I cleaned the master cylinder and flushed the lines. The pump works and you can feel the air blow out and suck in when the brake depresses. But.....when i add fluid, it will NOT pump fluid through the lines!! I've filled the lines full with a syringe and filled up the master cylinder; the lines are clear and the braked pumps air in and out, but not fluid.
Any help????

 

There is the possibility that your rear M/C will need a rebuild. Fluid may be flowing through your rear M/C plunger when pressure is present.