Mico hydraulic-over-air relay valves convert modulated input pressure to a proportional output air pressure and are suited for various on- and off-highway applications, such as controlling an air-braked trailer with an hydraulically braked towing vehicle having an air power source. An hydraulic vehicle component, such as a master cylinder or control valve, delivers an hydraulic input pressure that sends a pilot signal to the relay valve to modulate air brake pressure. Single, dual, and tandem valve designs are offered. The single input provides one hydraulic pilot port to control modulated air braking pressure. The dual features two independent ports that can accept pilot pressure, while the tandem input provides redundancy using two independent pilot pressure ports. Valves can accommodate a maximum air pressure of 150 psi (10 bar); maximum operating temperature is 250°F (121°C).