I never said he was wrong and yes I do understand the principles, but obviously you never saw my joke. Thank you.But it doesn't keep the pressure down, all you are doing is restricting the flow when the caravan is taking water. JTQ is exactly right.
Just as a means of showing you what happens is to open a mains tap just dribble, now block the outlet with your finger or thumb, and feel the pressure build up against your finger.
When its at its maximum, keep your finger on the outlet and turn the tap full on, you wont notice any change in the pressure.
Turn the tap off before you remove your finger.