I started having a similar issue the end of last season. Of course, I did nothing over the winter...
So anyways...maybe this will help you...or maybe you will help me....
I do not have any leak. If anything, it is overfilled from me thinking it must need fluid. When I remove the cap it leaks out.
Helm is only a couple years old. Fluid was replaced last year when I repaired the AP.
It is a very specific problem.
SOMETIMES, not always- When the motors are at Zero, dead ahead, if I SLOWLY turn the wheel it does not move the cylinder. I can very SLOWLY turn the wheel in one direction and the boat will not steer.
If I quickly move the wheel it works perfectly.
Often this can be made to happen by coming from hard over back to center and stopping there. If I
turn back from center, never a problem. But if from stopped at center, slowly
continuing the turn, it doesn' t move. Again, a little acceleration of the wheel and it works.
The problem only happens at center. Sometimes but not always.
It can happen coming from either direction.
If I pass thru center without stopping, it doesn't happen.
While I'm turning the wheel, there are never any noises associated with air in the system.
When the problem occurs it definitely feels like there is no pressure pumping to the cylinder.
When I turn it faster I feel the helm pump fluid.
I suspect it is the shuttle valve in the helm. or a spring/ball valve. maybe a speck of something in there
Just letting very low pressure fluid past a valve when at center..
I suppose it could be the cylinder letting some fluid pass at dead center due to a flat spot inside...maybe?
Maybe the AP? I suppose I could bypass that and test.
Of course the problem is that to do anything requires draining and removing the helm. To clean out the shuttle valve you need to replace the o-rings you destroy getting to it.
So at that point might as well have it serviced with all new o-rings.
I should have done that over the winter.
Seasick, I suspected the vented cap at one point. It never "vents" that I can tell. I was sure that was it. I put a fill nozzle hose rig in the filler and tested (so there was no cap on it). It seemed to fix it...
Then even with the cap back on it seemed fixed
..but later it was disproven. The problem had just gone away for a while because I was testing it...
I was going to buy a new vented cap just for shitzengiggles but they are sold in packs of 5 for $45.