The loop you see has a tap on it from mains pressure. Open the tap until the pressure reaches around 2 bar then close it again. If you go above around 3 bar it will vent off to the outside, so about 2 1/2 bar maximum before you turn it off. You may have a very slight leak somewhere that is losing the pressure. There is usually a rust mark or similar where the leak is.
Trev
August 2008
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