I recently bought a clarke pump model BPT600 to increase the pressure whilst filling my tank up in the back of my van. I followed the instructions in the leaflet to correctly assemble the pump, however when I plug it into my van, it cuts out after 40 seconds. However, if I switch it on whilst it is not plugged into my van, it does not cut out. This is the second pump I have had and both times I have encountered the same problem...Has anyone got any idea why this may be??