Can't bleed air out of rear brakes

When you say that you bench bled it; Did you in fact take it off the car, put it in a vise (nose slightely down), use the little fittings and hoses that usually come with the new m/c, and with the hoses submerged in the fluid, and stroking it full length and releasing the piston to allow it to return to the snapring stop?If yes and it wont quit sending bubbles then its probably defective. If you attempt to benchbleed it on the car(nose down) then you have to be sure that the piston is returning to the parked position. If it hangs up or the pushrod is improperly adjusted/seated then the compensating port may not open. In this case it will never bleed and neither will the next one. So;be sure the compensating port is open and working.