Never had that one...that is a puzzle.
I would first make sure nothing is dragging or getting caught on the steering mechanism behind the engine, including the control valve mechanism, which moves with the steering linkage.
I also wonder about the steering pivot bearings in the gimble. It sound good that you can steer lock-to-lock with the engine off. What would happen if someone stood on the outdrive (engine off of course), and you tried to steel lock-to-lock. It should be a harder to steer, but not extremely hard.
The control valve seems to be a simple and reliable device. Have not heard of having to replace one, but anythings possible.
Airlock does not seem likely, because a couple times lock-to-lock should have pushed all the air back to the reservoir.
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"Knot Easy" 2000 Horizon 240 Volvo 5.7GS /SX
tow: 2017 Honda PILOT EXL-AWD
prev. boats:
'87 Chaparral 198CXL 4.3 OMC Cobra
'69 Jetstar 16ft Ski Boat, 115hp Yamaha
'68 Aluminum Jon Boat, 3hp Sears
'64 Water Wings