Many times these systems will have a captive air bubble that stays in the system without notice until properly bled. I have had the autopilot pump move air into one of the helms causing it to cavitate. The easiest thing to do is pump the system up to 25 psi and crack one of the larger lines at the helm. Mind the pressure and don’t bleed it all off. Then keep turning the wheel hard until the rudder goes hard over. Once that happens keep cranking in that direction for several turns. What’s happening is the hydraulic line going to the cylinder that moves the rudder had the air in it. As you pressurize the line a relief valve opens sending the excess fluid and air back to the reservoir tank. Doing this several times in each di will purge the air. But you need to keep cranking when it goes hard over to open the relief love to purge the air.