Priming a hydraulic pump means filling it with fluid and removing trapped air before startup. Running a pump dry, even for a few seconds, can cause serious internal damage because hydraulic fluid serves as both the working medium and the lubricant for the pump’s internal components. The exact steps depend on your pump type and system layout, but the core principle is always the same: get fluid in, get air out, then start the motor.
Why Priming Matters
When a hydraulic pump runs without fluid, metal parts grind against each other with no lubrication. But the bigger threat is cavitation. When the pump tries to draw fluid and gets air instead, the pressure difference creates vapor bubbles inside the pump housing. These bubbles collapse violently, generating pressure waves that can exceed 10,000 psi. That repeated micro-impact erodes even hardened metal surfaces, leaving aggressive pitting that shortens the pump’s life dramatically.
You’ll know something is wrong immediately if priming fails. An unprimed or partially primed pump produces a distinct high-pitched whining sound. If air is being drawn into the system (aeration), you’ll hear something more erratic, sometimes described as marbles or gravel rattling inside the housing. Check the reservoir: if you can see the fluid, foaming on the surface confirms air is getting into the system. Any of these signs at startup mean you should shut down and re-prime before running the pump further.
Before You Start: Check Suction-Side Seals
Priming will fail if the suction side of the system has even a small air leak. Before you begin, inspect every fitting, O-ring, and seal between the reservoir and the pump inlet. A fitting that’s finger-tight or an O-ring with a tiny nick can let air bypass the fluid and keep the pump from filling. Tighten connections, replace any suspect seals, and make sure suction hoses aren’t cracked or collapsed. This five-minute inspection saves the most headaches.
Standard Priming for Gear and Vane Pumps
Gear and vane pumps are the most common types in industrial and mobile hydraulic systems. They have some self-priming ability, meaning they can pull fluid from a reservoir on their own, but that doesn’t mean you should just hit the start button on a dry system. Self-priming still takes a few rotations with little to no lubrication, and a fully dry startup puts unnecessary stress on the pump.
Here’s the general procedure:
- Disconnect power and relieve pressure. Make sure the system is fully de-energized and any residual pressure in the lines has been released.
- Fill the reservoir. Bring the hydraulic fluid up to the fill line. Use clean, filtered fluid that matches the system’s spec.
- Open the suction valve. If your system has a shutoff valve between the reservoir and the pump inlet, open it fully. For gravity-fed (flooded inlet) systems where the reservoir sits above the pump, simply opening this valve lets fluid flow down into the pump housing by gravity.
- Loosen the outlet fitting slightly. On some installations, you can crack open the discharge-side fitting just enough to let trapped air escape as fluid fills the housing. Once you see a steady stream of fluid with no bubbles, tighten it back up.
- Jog the motor. Instead of running the pump at full speed, bump the motor on and off in short bursts (a second or two each). This lets the pump draw fluid gradually without pulling a hard vacuum on a dry inlet. Listen for the whining or rattling sounds described above. If the pump sounds smooth after a few jogs, it’s primed.
- Run at low pressure. Once the pump sounds normal, let it run unloaded or at minimal pressure for a few minutes. This circulates fluid through the system and pushes out any remaining pockets of air.
Priming Piston Pumps
Axial piston pumps are less forgiving than gear or vane types. Their tight internal clearances mean they rely heavily on a film of hydraulic fluid between the pistons and the cylinder bore. Running one dry, even briefly, risks scoring those precision surfaces.
The critical extra step with piston pumps is filling the pump housing (the case) directly before startup. Most piston pumps have a case drain port, which is a separate line that returns internal leakage fluid back to the reservoir. To prime, remove the plug from the top case drain port and pour in clean, filtered hydraulic oil until the housing is completely full. While filling, let air escape from the port so you’re sure the housing contains no air pockets. Once fluid flows out with no bubbles, replace the plug and proceed with the jogging startup described above.
Skipping the case fill is the single most common mistake with piston pump startups, and it’s also the most expensive. A scored piston bore usually means a rebuild or replacement.
Priming a Non-Flooded System
In a flooded system, the reservoir sits above the pump, so gravity does most of the work. But many mobile and compact systems mount the pump at or above the fluid level. In these non-flooded setups, the pump starts empty and must create suction to pull fluid up, which makes priming more involved.
One reliable method uses pressurized air and a bleeder valve. You’ll need a second person for this. Fill the reservoir to its fill line, then open the bleeder valve on the pump or the suction line. Remove the reservoir lid, place a shop rag around an air gun nozzle to create a rough seal, and pressurize the tank with low-pressure air. This pushes fluid from the reservoir through the suction line and into the pump. Watch the bleeder valve: at first you’ll see air escaping, then a mix of air and fluid, then a steady stream of fluid. Once no more air comes out, close the bleeder valve and replace the reservoir cap. The pump is now primed.
An alternative for smaller systems is to pre-fill the suction hose with hydraulic fluid before connecting it to the pump. Disconnect the hose at the pump end, fill it with oil, then quickly reconnect it. This gives the pump enough fluid to begin generating suction on its own without running dry.
Cold Weather Priming
Low temperatures make priming harder because cold hydraulic fluid gets noticeably thicker. This increased viscosity means the fluid resists flowing through lines and into the pump, which creates the same conditions that cause cavitation: the pump tries to draw fluid, can’t get enough volume, and pressure at the inlet drops dangerously low.
If you’re starting a system in cold conditions, give the fluid time to flow. Gravity priming takes longer because thick oil moves slowly. Jogging the motor should be done in even shorter bursts, with longer pauses between them. Some systems use accumulators that store pressurized fluid specifically to reduce the load spike during cold startups. If your system has a reservoir heater, running it before startup to bring the fluid closer to operating temperature makes priming far more reliable. As a rough guideline, if the fluid is too thick to pour freely from a container, it’s too thick for a clean startup without warming first.
After Priming: What to Watch For
A properly primed pump runs quietly with a steady, low hum. In the first few minutes of operation, keep an eye on a few things. Check the reservoir level, because fluid is now filling cylinders, hoses, and other components throughout the system, and the level will drop. Top it off as needed. Watch for foam in the reservoir, which indicates air is still circulating. Listen for any return of whining or rattling. And monitor the fluid temperature: a pump that’s running hot shortly after startup may still have air trapped inside, since air compresses and generates heat in ways that fluid doesn’t.
If the system still sounds noisy after priming and a few minutes of low-pressure operation, shut down and re-check the suction side for leaks. A persistent air leak will undo your priming effort as fast as you completed it. Worn seals on the suction side are one of the most common sources of chronic air entry, and replacing them is the only lasting fix.