This fitting should always be mounted in the vertical upright position (shown below). In this condition, air flow is permitted to move through the unit in both directions (in and out). 
SPECIFICATIONS
Threads: 7/8-14 UNF (10AN ORB)
Barb (OD): 0.75" (19mm)
Material (valve): Stainless Steel
Material (retaining ring): Stainless Steel
Material (o-ring): FKM
Material (body): 6061 Auminum
Surface (body): Type-II Anodized