I was told it wouldn't work as a transfer pump as it needs pressure to start working. However, I charged it with mains water pressure, which must have 'primed' it, because it works really well as transfer pump now. I used it to hose off the van with pure water and it was really fast (gauge said about 50psi i think). If you put a tap on the end, it keeps building up pressure to about 60psi, then stops, until til you open it and pressure gradually drops.
Maybe you can adjust the cut off point on the pressure regulator valve, if it has one that's adjustable?
My mains is about 45-50PSi, so I don't use it as booster pump, just transfer. Fills barrels quickly!
I think I could use it to boost mains up to about 80PSi, but haven't had time to play, and it works well as transfer, and when I play about with things, they tend to leak etc, and becomes a major overhaul. All is tickety-boo at the mo, so will leave it.
Remamber Merlins don't like too much pressure, as they are only designed for small domestic water filtration use, whereas 4040 is capable of greater pressure, and works more efficiently when it is so.
Mind you, 20PSi is prettt dire isn't it?