So, following some communication with the steam boating forum we are all agreed that the Edwards Air Pump as drawn is missing  anything to hold the valve-plate in place, or seal its contact with the pump body. This means that:

1. The whole valve tends to move up and down with the piston rod, (and I think this needs to be a “good” fit to prevent leaking round the pump-rod, so this is unavoidable).
2. There is nothing to seal the lower valve plate (the one with the holes) where it sits on the ledge of the pump body…

So, my solution is to:

1. fit a gasket under the valve plate
2. make and fit a nylon block to hold the plate down – made 10 thou too long, so that the top cover provides some “squishing pressure”

As other members suggested this may help pump performance by removing “deadspace” (but it is beyond the valves so I am not sure if this is correct), and secondly it needed to be made in a way that ensured the outlet port is not cut-off if the block rotates; so, as can be seen in the pics below, I have made it with an exhaust annulus and internal ports to the valve chamber cut in the bottom…

I think this approach will also prevent the valve opening too far….. a test in the kitchen sink proves that it all works!

Here are some pics of the block and completed pump assembly.

air pump stuffing block in situ

air pump valve chest stuffing block

air pump valve chest stuffing block