[Nadar]

Yes, I'm aware of that, but as mentioned before after a couple of "pumps" the pressure builds up and the diaphragm/membrane doesn't return with the lever so that you can feel it's not actually pumping much. Opening the pipe connection into the injector pump releases the pressure and the diaphragm/membrane returns. That's why I suspect some kind of blockage, and the filter screen is a great candidate.