I'm testing pre-infusion manually like the picture in my Marzocco.
Recently the adjustment to swing became very hard.
What kind of problem is that?
How should I fix it?

See the "pressure adjusting screw" that's shown in the attached picture? Yours is just like that, except that it also goes through the "Acorn nut." If you're handy, you should be able to take these things apart and put them back together. If there's actually a problem, I'd guess that one of the pieces fell through a bit. That being said, it's possible it's supposed to be harder to turn. How much pressure are you getting out of the pump? Do you have a gauge for your portafilter to measure it? BTW, you probably already know, but most people say to never run those vane water pumps without water hooked up to them (I mention this since you'll likely be troubleshooting the part).

Thank you for your kind reply.
I change the pressure 4-9bars. I have a pressure gauge.
The pressure adjusting screw is more complicated than I have expected.
I appreciate your advice!