The steering system consists of three major components -- the cylinder, which moves the front wheels back and forth. It's double-acting, it has two hoses going to it, if one hose has more pressure than the other it will move toward the one with lower pressure. The steering valve, which is really a pump -- spin the wheel one way and it pumps one way, and spin the other and it pumps the other. The diverter valve, which detects if one of the hoses has a higher pressure (from the steering wheel being spun) and supplies fluid at pressure to that side and takes it away from the other, providing the power assist.
Here's what I see as the diagnostic point: the steering should work without the power assist. If you spin the steering wheel with the engine off and the front wheels don't move then something is amiss. It could be the cylinder bypassing, it could be the steering valve isn't moving fluid, or it could be the diverter valve is leaking into the rest of the hydraulic system.
If you're getting zero steering at all this isn't a subtle problem, so you can do some testing with the engine off. If you disconnect one hose from the steering cylinder and spin the steering wheel you can see if the steering valve is working at all. If you spin toward the disconnected hose you should get a stream of fluid out of it. If you spin away from the disconnected hose you shouldn't get any fluid out of the cylinder unless it moves. If the cylinder produces fluid without moving then it's bypassing. If spinning the wheel doesn't produce any fluid then the steering valve isn't working.
I wouldn't suspect air, it should be self-bleeding and all of the components are below the tank. But everything I know is self-taught...
