Aidan wrote:


On Dec 2, 11:59 am, Ian Stirling wrote:
Air goes through holes much, much easier than water, and will escape
hundreds of times faster than if filled by water.

True, but since it's compressible, the leak of a small volume of air
will cause a small reduction in pressure. For the same reason, you'll
be pumping away forever at the foot pump to get the system up to test
pressure.

Water is incompressible, so the leak of a small volume of water will
cause a large pressure loss, usually all of the pressure very quickly.

Oops, I completely forgot about that aspect.
If there is no pressure vessel, and no trapped air, then the only
pressure held in the system is by virtue of expanding metal, and
compressed water. So quite low volume to leak.