For anyone that is looking through this thread, there actually is an official procedure to isolate the master cylinder for fault. It's called "Brake Master Cylinder - Bypass Condition"
1. Disconnect the brake tubes from the master cylinder.
2. Plug the outlet ports of the master cylinder.
3. Lightly apply the brakes and hold for 10 seconds. Release the brakes and then reapply with heavy force. If brake pedal height cannot be maintained, the brake master cylinder has an internal leak and a new brake master cylinder must be installed.
After reinstalling the brake tubes, bleeding must be performed.
