The Primary Shutoff is the first line of defense to prevent the contents of the tank from entering the vacuum pump or blower. It may be labeled as Primary, Primary Shutoff, PSO, Float Shutoff or Ball Cage Shutoff, to name a few. Most Primary Shutoffs operate by a stainless steel float that seals against a soft seat. Some Float systems operate with a rod and pivot arm, much like a toilet float assembly. When the fluid level lifts the float to the point where air flow through the piping captures the float and draws it tight to the seat, air flow to the pump or blower is interrupted, preventing fluid from reaching the pump. Once the pump is shut off, the float is freed from the seat and may allow surging fluid to enter the pipping to the pump; this is handled by the next component; the Secondary Shutoff discussed below. Open this component monthly to be certain all parts are clean and functioning normally. While some tank manufacturers fabricate a propitiatory Primary Shutoff, most use assemblies made by one of the following companies: Masport, National Vacuum Equipment, Chandler Equipment, Fruitland, Westmoore, Metaltecnica (MZ), TTI and RIV.