Simon Richens is having trouble with his singing bird bellows.
I have viewed the video and it is clear that the reservoir is not
opening far enough.  This results in a low supply pressure and/or
insufficient supply volume.

It is not easy to see all details as the video is a bit blurred so
I cannot see whether he has set the spill correctly.  Bend the spill
lever so the valve opens only when the sprung board approaches the
cross member, i.e., allow it to go as far as possible when opening.
The spill lever should sit parallel to the sprung board and only a few
millimetres away from it.  I am assuming he has retained the correct
reservoir spring.

If this fails to improve the performance he has problems with either
the air-tightness of his bellows or the valves.  For the valves you
must work closely to Mason's specification and keep everything as flat
as possible.  Also check that the spill valve is sealing.  Recover if
in doubt.  Also check that your leather hinge for the pumping board
runs across the entire width of the board and seals against the zephyr
coverings, to prevent cross-leakage.

I know from personal experience that getting one of these to work
properly is very satisfying.

Good luck,
Nicholas Simons, GB