oil leak causes air in power steering fluid

Am I being told a story here or is this possible? I have a '03 Ford Escape that has developed a steering issue. A loud buzzing/humming could be heard when the car was being steered and accelerated. I thought the problem was with the wheel bearings. The mechanic is telling me that there is an oil pan gasket leak has cause an issue with the power steering pump belt that someway has caused air to get into the power steering fluid. The mechanic has topped off and bleed the power steering fluid and the noise goes away. However he has replaced the oil pan gasket and power steering pump belt. Do these repairs sound like they could cause the steering problem? I have not noticed any oil on my driveway and the same mechanic changed the power steering fluid 2 months ago. Is it possible that he just forget to bleed it then?
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