Learning as I go along, I decided to dismantle the pump (with the help of PDF manual no. 334599F and Graham from Hi-Tec Spray in Manchester on the phone). The pump was not drawing paint even though it was audibly running and it was still running even after the trigger of the G40 gun was released. In the past a similar problem was due to the ball valve at the bottom (the foot valve I think Graham called it, or ‘intake valve’ in the manual) having a sizeable piece of crud blocking it. This time I had to hunt a bit further and the problem seemed to be a similar issue at the upper ball valve or piston valve. I am giving the whole thing a thorough clean and what you see here is much cleaner than it started. We have been using exclusively water based (acrylic) paints and are just in the process of transitioning to ‘decorators’ paints’ - Leyland and Johnstone’s acrylic eggshell - to more suitable proper spraying paint. We chose Movac’s Sayerlack water based. We are spraying parts for fitted furniture, mostly white MDF. Cameo from Indigo the cat.