We tested the device again using the new Planar spring & new gaskets. Unfortunately, it still does not work.
The Planar spring is connected to the displacer using 3 screws. Unfortunately, the heads of these screws are too big. When the displacer pushes the spring upwards, the heads of the screws touch the bottom of the sealed chamber... This is a very precise device, any slight touching like this causes it not to work at all.
The problem is easily fixed (screws with a smaller head). However, I fear this is not the only problem. It seems likely that each time we fix one problem we may identify a new problem. But, eventually, we will get there !..