"Rod" wrote

The other day I replaced an old and leaking stopcock under the kitchen
sink - with a proper full bore valve.


Stop cocks are usually used not just as the house shut off valve, but also
for regulating flow around the house.
In replacing this with a full bore ball valve, you have removed this
regulation and are now subjecting your house mains pipework to full
pressure/flow from the external mains. This I think you have realised, as
you are turning down the flow at the external valve now.
Would have been better to replace the leaking stopcock with a new stopcock
to maintain the option to regulate, ball valves full-or-reduced bore are not
designed to control pressure/flow, they are for on-off duty only.

Phil