OT, the brass device in your pic is identical to a non return valve I have in an online air system in my Nissan. I use it to stop pressure backfeeding from my air tank through my air compressor.

The brass non return valve (NRV) is usually on the pumps output. It is there to protect the pump from the HP mains water supply. See this page - http://caravansplus.com.au/catalog/help ... system.php - scroll down to the heading - 4) Adding Mains Pressure and Hot Water

Yep, that is exactly what I stated

when we bought our new (to us) van, it was fitted with an in-line pump.
The down fall of these is that they are not self priming, but it helps us conserve water

When we only need a drink we use the hand pump, when we need to fill the kettle we use the hand pump to prime then flick the switch and the pump takes over saving 30 or 40 pumps.
Being an in-line pump it is not connected to the van at all but hangs under the sink and therefore does not make much noise only the hum of the pump where as the pressure pumps seem to reverberate through the van

Our water pump will come on after some time, i guess it loses pressure. We have a switch next to the sink and turn that on and of when we want to use the tap.

Hans