What is the function of a relief valve in a hydraulic system?

• A. To limit system pressure by allowing fluid to flow back to the reservoir when set pressure is exceeded
• B. To increase system pressure
• C. To vent air from the system
• D. To filter contaminants from the fluid

Answer: (A)

Relief valves are safety devices that keep a hydraulic system from overpressurizing. They’re set to open when the system pressure reaches a chosen limit, and once open, they let fluid flow back to the reservoir. By shunting fluid away from the circuit, they prevent pressure from climbing higher, protecting pumps, hoses, seals, and actuators from damage. When pressure drops, the valve reseals and normal operation resumes.

This isn’t about increasing pressure, venting air, or filtering fluid—those would be handled by other components (a pressure-boosting path, an air vent or breather, and a filter, respectively). So the primary role of the relief valve is to cap the system at a safe maximum by redirecting excess fluid back to the reservoir.