ThomasKlin wrote: One of the biggest failures was with the original idea I had of creating a non-return valve system that would allow pouring the water without having to lift the piston out of the cylinder. It is probably possible to do, but I couldn't make it work by simply using inexpensive and readily available parts such as silicone sheets and washers.

Have you considered making it in the shape of a bicycle pump? You'd want to cut out two opposite semi circles from the bottom piston.

So that the seal can flex inwards when pulling up and let the water in.

Actually, come to think of it - it won't work as it depends on a vacuum being created in the piston to force the Oring to flex and let the air or water in. In a bicycle wheel there is another one way that prevents the air from going back into the piston.

The only other way is to have the plunger move freely in the piston (think small screw in a large hole). It mends to be large if vacuum is not to form on the bottom of the piston. Water can flow unrestricted until pressure is applied. Sealing a large surface are at 9 bar may be hard....

Anyway just a few thoughts.

NB. What is the pressure in a pressure assisted flush toilet? Flushmate does not seem to spec it so my best guess is that it is about 3 bar (being the mains pressure stored against a sealed chamber. Can that be used for espresso?

Actually, the bike pump system is what I had in mind at the beginning, not quite like what you described but the same principle. I think this is definitely possible, is just in my case too many things were not working as I though they would and in the end I gave up.
Have a look at the sketch below, this was the original plan.

Yup. That's it. The bottom one way is needed. It would be a pain to clean...

maybe 40 pounds in monetary terms, but for me it represents ages of engineering and machinery experience. i wish i can do something like that. up until now, i will probably order that leverpresso