350SL Fuel Vapor System

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Question for the experts:

My 1972 USA model 350SL has a fuel vapor recovery system. When I remove the fuel cap, a new MBZ locking cap for this model, it pops off like the tank is under pressure, and there is an audible hiss as the pressure releases. Is this normal? It seems that there should be a slight negative pressure (vacuum) in order to keep the flow out-to-in rather than have higher pressure inside that could force fuel vapors out the filler neck. Where am I wrong in my thinking?

Thanks,

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