It is normal for there to be a hiss from the pressure switch for a second when the unit stops, this allow any pressure in the line to be dissipated so next time the unit starts it doesn't have a pressure to start against. If it is constantly leaking it will be a problem with the non return valve. It is normally a brass fitting on the tank that the compressor pipe goes to. It may just have a bit of debris not allowing it to close properly, You need to drain down all air and remove the cap on the brass fitting, there should be a rubber shuttle with a spring behind, if it is hard an cracked it will need replacing or it may just need a clean.

Hope this helps,

James