In an earlier contribution to this discussion,
Ian Stirling wrote:

Otherwise, a T above the existing valve, followed by a valve will
work, sort-of.
(as there is no means of admitting air, so the pipe may take a while
to drain.)

If you run off some hot water - after turning off the cold - the tank level
will drop a bit, allowing the ball valve to drop. This will admit air. [I
suppose you could just force the ball down with a piece of wood to avoid
having to drain any hot off]
--
Cheers,
Set Square
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