Sounds about right!
Even the industrial ones are never considered to be 100% leak tight
and the conservative approach is to put in two of different types and
then you can assume that the backflow is only 10% of the rated forward
flow.

That said, in many domestic situations you can probably tolerate a
liitle bit of back-flow (trickle?) as the bulk of the flow should be
stopped.

HTH,

Tim.
(ps McAlpine was the name i was thinking of, have just binned one of
their catalogues along with a load of other stuff)

On Fri, Jul 3, 2009 at 11:00 PM, Chris Hunter
(home)<chris.cjcmhunter@...> wrote:
> we could do with one, too ... we bought a couple of (IIRC) McAlpine
> (Sp?) ones, and found they let a trickle through when closed, via the
> hinge arrangement  ...
>
> Chris