On Wed, 17 Dec 2008 17:16:40 +0000, Invisible Man
wrote:



Same type of principle as rocking a car back and forward to get it out
being stuck in a muddy or snowy hole. Pressure from supply stops when it
is turned off and water with air trapped will move a bit. When supply
turned back on again pressure shifts it a bit and so on.

Nice analogy. Thank you. I can see that.
I've never known of an air lock in mains pressure pipe before.
Live and learn!
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Ian