Power steering issue


My 03 E500 sat for a while (about a year) and when I started it up, everything worked well. Not long later, turning of the steering wheel felt hard. The power steering fluid reservoir is topped off to the recommended level
When I turn the wheel, there is no humming noise like you hear when there is no fluid, except when I turn it to either the extreme right or extreme left. Turning is very tight/stiff but seems like there is a little assistance from the power steering pump. It's not as tight as if it wasn't working at all. Any suggestions in what I might try prior to replacement of the pump?