This might just work
Spoiler - it did.
ON THE WAY!
Print all the parts. PLA is good, 50% infill, plenty of shells, 100um layers, tiny nozzle (0.25mm recommended) Not all parts are proprely oriented for printing No supports are required, except for the valve flappers
Assemble valves to main body using 1.75mm filament pegs for alignment and rtv silicone around o-ring to seal, superglue to secure
Use 1mm o-ring glued into piston grooves and along apex. Go slow and make sure it's flat and consistent.
MORE ON THE WAY!