1996 Mercury Sable

I have a 1996 Mercury Sable 3.0 DOHC 24 Valve
110,000 mileage. When I shut the engine off after driving a while. I here a hissing sound coming from the power steering reservoir and the power steering fluid leaks out of the top of the reservoir around the cap. It only happens once in a while and after a couple of time I have to refill the reservoir. Is it supposed to have that much pressure or could it be that the o-ring on the cap is worn? Or is there something else wrong?
July 17, 2006.

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