Mark, its possible that you have a leak in the pipe before the check valve. However if the pump isnt running then that wont be the proble. If you look at the diagram above then you can see 3 separate washer pumps, each one controls either the windscreen headlamps or rear screan. with the bonnet up get someone to try the windscreen washers, and the rear screen you should hear the pumps then. sounds to me like a pump has gone down.