The P1212T in-tank style check valve has 1.25" male pipe thread (MPT) that threads into the receiver tank and has 1.25" female pipe thread (FPT) on the top where typically a discharge line from the piston compressor pump would connect.
This is constructed of a one piece brass body with stainless steel springs. Unlike lower quality valves, this has a glass filled fluoropolymer poppets for long yerm reliability. If there is ever a leak from the pressure switch this valve can be disassembled for cleaning and repair. They are 100% tested at the factory for backflow leakage performace to ensure proper operation.
A 1/8" threaded NPT port is included to connect an unloader line that may route back to your pressure switch. Otherwise this can be capped off using the 1/8" pipe plug.
An air compressor check valve is designed to keep high pressure air inside the receiver tank so the compressor is not fighting backpressure when it starts up. A faulty check valve can cause excess wear and tear on the motor and may also cause damage to the pump