I don't know that air in the system would cause it to be "stiff", generally it will feel kind of mushy, or skip a bit when you turn the wheel. You might be having problems with one of the check valves depending on the make and model of pump. I had a Wagner pump blow out on us not too long ago, it blew one of the threaded caps right out the side of the pump (and a LOT of fluid of course). Before it went it had started to feel "stuck" when trying to turn back from hard over.
SV Laurie Anne
1988 Brewer 40 Pilothouse