Passenger side of the reservoir. To check for internal leaks, pump the brakes until the pump cylces and then shuts off. Turn off the ignition. Push a clear plastic hose, which is cut square on the end, over the return port (the rear port). If brake fluid rises in the tube, there is an internal leak.
To test for a leaking check valve, press the tube over the supply port ( the front port). If fluid rises in the tube the check valve has a leak.