What is the purpose of a hydraulic reservoir drain and bleed procedure?

• A. To remove air from the reservoir only.
• B. To remove old fluid, contaminants, and air from the system and ensure proper filling and pressure.
• C. To replace the reservoir with a new one.
• D. To diagnose electrical faults.

Answer: (B)

Purging the hydraulic reservoir and the connected lines is all about removing old fluid, contaminants, and trapped air from the system and then refilling to the correct level so the system can develop and maintain proper pressure. Old fluid can break down seals and create varnish or sludge; contaminants can clog valves and orifices; air in the hydraulic path compresses, leading to spongy controls, delayed response, or loss of pressure. By draining, flushing or refreshing the reservoir contents, and then bleeding through the system to remove air from all circuits, you ensure the fluid is fresh, clean, and that no air pockets remain in the lines or actuators. This sets up the system to operate predictably and safely with the right pressure.

It’s more than just removing air from the reservoir; you’re addressing the whole circuit—old fluid and contaminants are also removed, and the system is filled to the correct level with the proper hydraulic fluid, then bled to verify and establish the correct pressure. Replacing the reservoir or diagnosing electrical faults aren’t what this procedure accomplishes.