Priming a hydraulic pump means filling its internal housing and suction line with fluid before startup so the pump never runs dry. Running a hydraulic pump without fluid, even briefly, causes metal-on-metal contact and can destroy internal components within seconds. The process is straightforward, but the details matter depending on your pump type and conditions.
Why Priming Matters
Hydraulic pumps are designed to move fluid, not air. When air pockets remain inside the pump housing at startup, those bubbles get compressed as they move through the pump and then violently collapse. These implosions create intense shockwaves that pit and erode the internal surfaces of the pump. Over time (or even in a single dry start), this damage chews up gears, vanes, or pistons and destroys the pump housing.
Beyond component damage, trapped air also prevents the pump from building proper pressure. You’ll get erratic flow, noisy operation, and a system that can’t do useful work until all that air is purged.
Before You Start
A few things need to be in order before you touch the pump.
First, verify the reservoir oil level. The fluid should sit at least three inches above the pump suction inlet. If the level is too low, a vortex can form in the tank and continuously pull air into the suction line, making priming impossible. Top off the reservoir with the correct fluid grade before proceeding.
Second, check the suction line for any obvious issues. A clogged suction strainer inside the tank, a missing breather cap on the reservoir, or a loose fitting anywhere on the suction side will let air into the system and defeat your priming effort. Inspect every connection from the reservoir to the pump inlet.
Third, consider fluid temperature. Cold hydraulic fluid is thick, and thick fluid resists being pulled through suction lines. Most hydraulic systems operate best with fluid between 30°C and 70°C (roughly 85°F to 160°F). In cold environments, the fluid’s viscosity can climb high enough that the pump simply can’t draw it in. If you’re starting up in freezing conditions, you may need a lower-viscosity fluid grade (such as VG 22 for arctic conditions or VG 32 for winter) or a way to pre-warm the oil.
Finally, make sure any stored hydraulic pressure in the system has been safely relieved before you open fittings or drain ports. OSHA requires that residual energy in hydraulic systems be bled off before servicing. Wear eye protection and gloves, since hydraulic fluid under pressure can penetrate skin.
Step-by-Step Priming Procedure
The core process is the same across most pump types: fill the housing with clean fluid, eliminate trapped air, then start the motor carefully.
Fill the Pump Housing
If your pump has a case drain line (most piston pumps and many vane pumps do), disconnect the top case drain port and use it as your fill point. Pour filtered, clean hydraulic oil into this port slowly. As you fill, air will escape from the housing. Keep adding fluid until oil flows steadily from the port with no bubbles. Then reconnect the drain line.
For gear pumps or pumps without a case drain, you can often remove a plug on the top of the housing or disconnect the suction line at the pump inlet. Fill the housing and suction line with fluid directly, then reconnect everything. The goal is the same: get fluid into every internal cavity so no air pocket remains when the motor spins.
Jog the Motor
Don’t just flip the switch and let it run. Instead, jog the electric motor in short bursts, a second or two at a time. Watch your pressure gauges to see if suction and system pressure begin to build. Each jog pulls a small amount of fluid into the pump and pushes air out. After a couple of jogs, you should see the gauges respond. If pressure doesn’t build after several attempts, stop and recheck your suction line and fluid level.
For diesel or engine-driven pumps where jogging isn’t practical, keep the pump at the lowest possible RPM during initial startup and let it run unloaded (no pressure demand from the circuit) while it primes.
Run Unloaded
Once the pump picks up fluid and pressure starts to register, let it run at low speed with no load for several minutes. This gives the pump time to fully evacuate any remaining air pockets and reach a stable operating temperature. Listen carefully during this phase. Smooth, steady operation means the pump is primed. Any knocking, whining, or rattling means air is still present.
Bleeding Air From the System
Priming the pump is only half the job. Air trapped in the lines, cylinders, and valves downstream also needs to come out, or the system will operate sluggishly and erratically.
Locate the bleed valves on your system. These are typically small valves at high points in the circuit where air naturally collects. Open each bleed valve by turning it counterclockwise. Leave it open until the fluid flowing out runs clear with no visible bubbles, then close it fully. Work from the farthest point in the circuit back toward the pump. This sequence pushes air progressively out of the system rather than trapping it between closed valves.
If your system doesn’t have dedicated bleed valves, you can crack open fittings at high points in the circuit to let air escape. Tighten them as soon as clean fluid appears. Cycle the actuators (cylinders, motors) slowly through their full range of motion a few times to push trapped air back to the reservoir, where it can separate from the fluid naturally.
Differences by Pump Type
Gear pumps and vane pumps almost always need manual priming before their first use. They rely on tight internal clearances to create suction, and those clearances don’t seal well against air. Fill the housing and suction line with fluid before startup, every time the pump has been drained or replaced.
Axial piston pumps are more sensitive to dry starts because their pistons and valve plates run at extremely tight tolerances. These pumps typically have case drain ports, which makes filling the housing straightforward. Parker’s commissioning guidelines for piston pumps specifically recommend jogging the motor and watching for pressure buildup at both the replenish (charge) circuit and the servo circuit before allowing full-speed operation.
Some internal gear pumps and certain specialty designs are self-priming, meaning they can evacuate air from the suction line on their own through a built-in jet pump mechanism. If your pump is rated as self-priming, you still want to fill the housing with fluid before the first start, but the pump can handle re-priming after brief shutdowns without manual intervention.
Signs the Pump Isn’t Primed
A pump running with air inside it tells you clearly. The most common symptom is a knocking or rattling sound, often described as marbles bouncing around inside the housing. This is the sound of air bubbles collapsing against metal surfaces. If you hear it, shut the pump down immediately and re-prime.
Other signs include erratic pressure readings, foamy or milky oil in the reservoir (indicating air is mixing with the fluid), excessive heat at the pump, and sluggish or jerky actuator movement. Foamy oil is especially telling because it means air is being continuously introduced, not just trapped from a one-time event. That points to an ongoing suction leak rather than an incomplete prime.
To find a suction-side air leak, inspect every fitting, hose, and seal between the reservoir and the pump inlet. Even a tiny gap in a threaded connection or a cracked hose can pull air in, since the suction side operates below atmospheric pressure. Check that the reservoir breather cap is in place and that any suction strainers inside the tank aren’t clogged. A blocked strainer starves the pump and mimics the symptoms of air entrainment.
Cold Weather Priming
Cold temperatures make priming significantly harder. Hydraulic fluid thickens as it cools, and once viscosity climbs above roughly 800 mm²/s, most pumps can’t pull the fluid through even a short, straight suction line. At that point, the pump cavitates regardless of how well you pre-filled the housing.
If you’re operating in cold conditions, use a fluid grade matched to the temperature range. Arctic-rated fluids (VG 22) remain fluid well below freezing. Standard mineral oils with a pour point around negative 30°C will turn solid in extreme cold. Bio-based fluids and polyglycol-based fluids have their own low-temperature limits and need careful selection. When in doubt, pre-warm the fluid in the reservoir with an immersion heater before startup. Getting the oil above 30°C before priming makes everything easier and dramatically reduces wear during the first few minutes of operation.