Brass check valve for reciprocating air compressors, with two male threaded ports set at right angles and a small compression-nut port for the unloader line. It holds receiver pressure when the compressor stops and lets the pump head bleed off.
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The valve is screwed between the compressor pump discharge and the receiver. While the compressor runs, air passes into the receiver; when the motor stops, the valve seats and holds the receiver pressure so the pump does not have to restart against a full tank. The small side port, fitted with a compression nut for a small-bore line, is the unloader take-off: it vents the trapped air between the check and the pump head so the motor restarts unloaded.
The body photographed is brass, with the two main ports male-threaded and set at right angles to each other — one to the pump discharge, one to the receiver — and the unloader port on the top face. Nothing is cast or stamped on it: no maker, no thread designation and no pressure rating.
Because compressor fittings are made in both BSP and NPT and in several sizes, the critical ordering information is the thread form and size of each of the three ports, measured off the compressor and the receiver boss, together with the outside diameter of the unloader tube. BSP parallel (ISO 228-1), BSP taper (EN 10226-1) and NPT (ASME B1.20.1) are not interchangeable.
| Valve Type | Check valve for reciprocating air compressors, with an unloader take-off port |
|---|---|
| Body Material | Brass |
| Ports | Two male threaded ports set at right angles (pump discharge and receiver), plus one small side port with a compression nut for the unloader line |
| Thread Form and Size | Not marked on the photographed unit — measure the compressor and receiver bosses and specify BSP parallel (ISO 228-1), BSP taper (EN 10226-1) or NPT (ASME B1.20.1) with the size for each port |
| Unloader Connection | Compression nut for a small-bore unloader tube; state the tube outside diameter when ordering |
