My system is hynautic with a raymarine autopilot pump installed near thevupper helm.
I have had the exact symptoms for 7 years and 15000 miles.
One year I noticed a tiny leak in a fitting near the rudder ram and tighten, then filled snd pressurized.
Over a 5 month summer no cruising oeriod, the pressure will bleed down from 30 to 15 psi say.
Other than that, I have found no external problems.
While cruising, after 3 or 4 days, the upper helm does get loose and maked the autopilot oump work harder ir seems. Especially to starboard. So I go up and spin the wgeek left and right till it firms up and is 5.5 turns stop to stop like the lower.
The lower never seems to have the problem.
So I live with it as rebuilding the upper may be out of the question due age...plus it hasnt gotten high enough on the worklist yet.