How to Bleed Air from a Hydraulic Pump: A Step-by-Step Guide
At Shijiazhuang Baile Electromechanical Equipment Co., Ltd., we specialize in the professional production of high-performance hydraulic pumps, hydraulic motors, and hydraulic valves. Every day, we work closely with engineering teams and maintenance technicians worldwide to ensure their fluid power systems run at peak efficiency.
One of the most critical, yet frequently overlooked, maintenance procedures is removing trapped air from the system. In this guide, we will share our professional expertise on how to properly bleed air from a hydraulic pump to protect your equipment and prevent costly downtime.
Why Bleeding Air Is Critical for Your System
A hydraulic pump is designed to displace incompressible liquid, not compressible gases. When air becomes trapped inside the pump housing or suction line, it leads to a destructive phenomenon known as aeration or cavitation.
If left unaddressed, trapped air can cause:
Severe Internal Damage: Air bubbles collapse violently under high pressure, micro-detonating against metal surfaces and eroding internal components.
Erratic Performance: Sluggish, jerky actuator movements and an inability to maintain stable system pressure.
Excessive Noise and Heat: High-pitched whining or knocking sounds accompanied by a rapid rise in fluid temperature, which degrades seals and breaks down oil viscosity.
Whether you have just installed a brand-new pump or completed a routine valve replacement, bleeding the air out before running the system under load is absolutely essential.
Step-by-Step Guide to Bleeding a Hydraulic Pump
Before beginning, always prioritize safety. Wear appropriate personal protective equipment (PPE), ensure the system is completely depressurized, and clean the work area to prevent external contamination from entering the hydraulic circuit.
Step 1: Check and Fill the Reservoir
Before turning on any equipment, ensure your hydraulic reservoir is filled to the maximum recommended level with clean, high-quality fluid. A low fluid level will create a vortex, pulling more air directly into the pump inlet.
Step 2: Pre-Fill the Pump Case (For Piston Pumps)
If you are commissioning a variable or fixed displacement axial piston pump, you must pre-fill the pump housing.
Remove the highest plug on the case drain port.
Pour clean hydraulic oil directly into the pump case until full.
Reinstall the plug securely. This provides vital lubrication to the pistons and swashplate during the initial start-up.
Step 3: Loosen the Outlet Fitting (Cracking the Line)
To give the trapped air an escape route, slightly loosen (crack) the fitting on the pump's pressure outlet port or the bleed valve if your system is equipped with one. Do not remove the fitting entirely-just loosen it enough to allow air and fluid to seep out. Wrap a clean rag around the fitting to catch any fluid spray.
Step 4: Jog the Prime Mover
Do not start the engine or electric motor and let it run continuously. Instead, jog (bump) the starter for 2 to 3 seconds at a time, then switch it off. Repeat this process several times. This action turns the pump over slowly, pushing air out of the loosened fitting without running the internal components dry.
Step 5: Seal the Line When Flow Stabilizes
Watch the loosened outlet fitting carefully during the jogging process:
Initially, you will hear sputtering air and see foamy, milky oil.
Once a steady, solid stream of clear hydraulic fluid emerges without bubbles, the pump is primed.
Immediately tighten the outlet fitting back to the manufacturer's torque specifications.
Step 6: Cycle the System Under No Load
Start the prime mover and let it run at idle or low speed. Slowly cycle your hydraulic valves to extend and retract cylinders or rotate hydraulic motors. Operate the entire system under zero or minimal load for a few minutes to sweep any remaining air bubbles out of the lines and back into the reservoir where they can naturally dissipate.
Finally, shut down the system, re-check the reservoir fluid level, and top it off as needed.
Partner with Shijiazhuang Baile Electromechanical Equipment Co., Ltd.
Proper maintenance keeps a system running, but long-term reliability starts with high-quality engineering.
Shijiazhuang Baile Electromechanical Equipment Co., Ltd. is a premier, professional manufacturer dedicated to producing world-class hydraulic pumps, hydraulic motors, and hydraulic valves. Our extensive product lines are built using premium materials and precision manufacturing techniques, ensuring high volumetric efficiency, exceptional pressure resistance, and an extended operational lifespan.
Whether you are upgrading an industrial production line, servicing heavy construction machinery, or designing agricultural equipment, we supply the reliable fluid power components you need. We don't just sell parts; we provide comprehensive technical support to help you install, bleed, maintain, and optimize your hydraulic systems for the long haul.
Need dependable fluid power components? Contact Shijiazhuang Baile Electromechanical Equipment Co., Ltd. today to discuss your technical specifications and experience the difference that professional manufacturing makes.
