There should NOT be any + or - pressure inside the rear diff,that is why it is called a vent and not a valve,it is there for equal pressure hot or cold.If it hisses either hot or cold it is plugged and needs to be removed and cleaned in solvent.
It's a one way valve with a spring and a rubber seal. Trust me. I took mine apart when I put on the turkey baster version.
2012 V6 Base. 6-speed. 3.31. Roush Wheel Hop Kit. BMR 38mm Front Swaybar. Camber Modded Koni Yellow Struts. Linear Steeda Sport Springs. J&M Camber Plates. J&M Panhard Bar. Airaid Intake. 93R Bama Tune. BBK Ceramic Shorties. Carbon Fiber Trac-Lok w/ Finned Cover. Hawk SR pads. GT Rotors. Spec Al Flywheel & Stage I Clutch, 18x10.5 ET38 Enkei PF01 Wheels. 285 30 R18 RE-71r tires. Boss 302 Radiator.