On 22/08/2012 11:49, Dave Liquorice wrote:
On Wed, 22 Aug 2012 11:40:11 +0100, Frank Erskine wrote:

Removing this screw reveals a small O ring around the screw and hole
to the water inlet.

But what is it for?

Perhaps to drain the tap/pipe for cold spells.

You could just open the tap. But some Bib taps have a built in double
check valve this could be to drain the bit between the two valves.

That would make sense although I have a feeling that one did have a
non-return valve and the other did not.

It was the failure of the non-return bit that prompted its replacement.
A little O ring that seals it closed was coming loose and I was bu**ered
if I could find a way of getting it all apart to replace it.