Relay valves are electro-mechanical devices used primarily within air brake and air suspension systems of heavy-duty vehicles, and increasingly in some passenger car applications. They translate a low-pressure signal from the control valve into a higher pressure output to actuate pneumatic components like air bags or brake chambers. Functionally, they operate as pressure amplifiers, requiring minimal electrical current for significant pneumatic force. Internal construction typically includes a solenoid-operated piston and valve assembly, with calibration adjustments for precise pressure regulation. Common features include porting variations for single or dual circuits and materials designed for durability in harsh environments.