“An o-ring pump head would be a great idea, to test the difference. The problem might still be the cup not sealing well enough. A hint towards this theory is that pumping becomes harder up to 8 or 9 pumps, but then it's not that much more difficult to go 12 pumps, and the velocity increase is marginal. Maybe the pump is then simply not coping with the pressure, and leaking air - without me hearing it. “

That doesn’t sound right, should be harder to pump with each stroke?