When I've had a problem like this in the past, I start with the basics and work rearward. First, plug off both outlets on the master and REMOVE ALL VALVES from the entire system. Pump the brake and hold it for 30 seconds. Do you have a hard pedal? If no, then the master is bad. If yes, then the master is good and you can move on to the next test. Hook up the front calipers, leaving the plug in the master where the backs hook in. Pump the brake and hold it for 30 seconds. Do you have a hard pedal. If no, then the front calipers are at fault. If yes, reconnect the rear brakes and pump and hold the brake for 30 seconds. Do you have a hard pedal? If no, then the rear calipers are at fault. If yes, then one of the valves are at fault.