Cause of a resevoir power steering fluid leak

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I have power steering fluid leaking from around my power steering resevoir that the mechanics can't find the source of the problem. I just had new power steering hoses put on, and last june I have my power steering pump replace. The fluid is coming out of the resevoir, but they can't find a leak. I can drive if for about a week before I have to put more fluid into the resevoir. When it gets low my accelerator make a sound like an electric car when I push on the gas pedal. Do I need to take it to the dealer? It has never had any major problems, only routine maintenance, and has 104,000 miles on it.