Hydraulic Pump Start-Up Procedure After a Rebuild or Replacement

Fill the pump case with clean, filtered oil through the highest case drain port, fill the inlet line, jog the motor to confirm rotation, then start the pump unloaded (relief and compensator backed off) and let it purge air at low pressure before you load it up in steps. Every factory start-up procedure we keep on file, from Vickers and Parker to Rexroth, Danfoss, Oilgear and Kawasaki, follows that same order; what changes from pump to pump is the detail, and the detail is what this page is about.

A scored rotating group, a shaft seal blown the first afternoon, a pump that "won't pump" because it is spinning backward: almost every bad first start is avoidable with ten minutes of preparation.

The step that kills the most rebuilt pumps: starting with a dry case. Kawasaki's K3VL manual puts it bluntly: filling only the suction line is not enough, because the bearings, pistons, shoes and spherical bushes need oil from the first revolution, and seizure follows very quickly if the case is dry. Every piston pump manual we have says the same thing in different words.

What is the correct start-up order for a hydraulic pump?

Condensed from the factory procedures in our sources, with a printable checklist at the end:

Find out why the old pump failed, and fix it. It is step one of Continental's HPV replacement checklist. A new pump in a contaminated or mis-plumbed system dies the same way the old one did.
Clean the system. Drain and clean the reservoir, flush lines and components, install a new filter element and fill only through a filter.
Fill the pump case through the highest case drain port with clean, filtered oil.
Fill the inlet line and open every suction valve.
Unload the circuit. Back off the relief valve and the compensator, or put the pump in neutral or free circulation.
Jog the motor and check rotation against the arrow or the model code.
Start and purge air at low pressure until the pump primes and the oil runs clear.
Raise pressure in steps, watching case drain flow, case pressure, temperature, noise and leaks.
After run-in, change the filter element, top up the reservoir and record a baseline case drain reading.

Why does the pump case have to be filled, and through which port?

On a piston pump the case oil lubricates the shaft bearing, the slippers and the shaft seal. The case only fills by internal leakage once the pump is running, so a dry case runs metal on metal for exactly the window in which a new valve plate or shoe set gets scored.

The factories are consistent about how to do it:

Parker says the PV series case must be filled through the drain port to ensure lubrication, sealing and a smooth start, and that the drain must come off the highest drain port, above the pump centerline, or an extra air bleed line is needed (bulletin HY30-3245-INST).
Rexroth's A10VSO data sheet (RE 92711) requires the unit to be filled and air bled at commissioning and again after any long standstill, because the case can drain down through the lines. Drain from the highest tank port (L or L1). In installation positions where complete filling is not possible, Rexroth says to fill and bleed the pump horizontally before installing it.
Danfoss (Series 90 service manual 520L0818) fills through the upper case drain port and has you open the case plug on top of the control to let the air out.
Oilgear's PVM service bulletin 947070-B says to remove the case drain line, fill the case, then turn the shaft a few times by hand to make sure the parts rotate.
Vickers (PVH overhaul manual M-2210-S) stresses filling before start-up, particularly when the pump sits above the reservoir oil level.

Use the system oil, poured through a filter, until it comes out of the port, and plumb the drain from that same top port so the case stays full at shutdown. Oilgear's troubleshooting chart lists a low case oil level from using the wrong drain port as a cause of overheating.

How much oil does a pump case hold?

Few manuals publish it. Kawasaki does, which makes a useful reference for the K3VL family:

Kawasaki model	Case fill capacity
K3VL28, K3VL45, K3VL60	0.6 L
K3VL80	0.8 L
K3VL112, K3VL140	1.4 L
K3VL200	3 L
K3VL200H	3.2 L

For any other pump, fill until oil reaches the drain port. Do not guess a volume and stop short.

How do you prime a hydraulic pump and bleed the air out?

A pump primes when the inlet is flooded and the air in the pump and outlet line has somewhere to go. Most "won't prime" calls come down to one of those two.

The inlet side

Fill the suction line, and make sure every suction gate or ball valve is open. Parker recommends a shut-off valve in a flooded inlet be monitored by a proximity switch so the motor cannot start with the valve closed.
Every inlet fitting must be tight. Continental singles out the suction piping because an inlet leak draws air instead of leaking oil, so you will not see it.
The reservoir has to be above the suction pipe opening with room to spare for the oil the system will soak up as it fills.

The outlet side

Give the air an exit. Vickers' V-series vane manual (M-2000-S) says a pump driven at 600 rpm or more should prime almost immediately if the air can be purged, which may mean cracking an outlet fitting, and warns that a slow prime can damage the pump through lack of lubrication.
Know what "a few seconds" looks like. Vickers expects a PVH to prime within a few seconds; if not, check the inlet for restrictions and air leaks and make sure air can escape at the outlet. Once primed, tighten the outlet and run unloaded for five to ten minutes.
Jog it. Parker runs the PV in short bursts at free circulation until the lines are full and bubble-free; Oilgear switches the drive on and off several times before letting the pump reach full speed.
Bleed the high points. Oilgear says to loosen connections or open petcocks at the highest points in the system and close them when a solid stream of oil appears.

When the oil in the sight glass is clear rather than milky, the air is out. Vickers uses exactly that check in the PVH start-up section.

New hydraulic pump not pumping?

Run down this list before you pull the new pump back out:

Symptom	Usual cause
No flow at all, pump quiet	Wrong rotation, or a closed suction valve
No flow, pump noisy or rattling	Air leak on the suction side, low reservoir, or inlet restricted
Flow but no pressure	Relief or compensator still backed off, or air trapped against a closed-center valve with nowhere to vent
Primes, then loses it	Inlet fitting drawing air, or a suction line too small or collapsed
Vane pump will not pick up	Drive speed too low; Vickers' vane priming figure is 600 rpm minimum

Continental's HPV troubleshooting table lists the same three culprits for "pressure does not rise": wrong rotation, low tank level and a clogged suction line. For deeper diagnosis, see hydraulic pump not building pressure.

How do you check hydraulic pump rotation?

Most piston and vane pumps are built for one direction only. Parker says so plainly for the PV series and tells you to check the drive motor's rotation before installing the pump. Oilgear's rule is just as simple: a clockwise pump is driven clockwise, a counterclockwise pump counterclockwise, and the rotation plate on the housing tells you which you have.

Read the rotation from the housing arrow or the model code (decoders: Vickers, Parker, Rexroth A10, Denison, Sundstrand).
With the pump uncoupled if possible, bump the electric motor and watch the coupling. Three-phase motors are wired either way, so never assume.
Couple up, fill, and jog once more. Kawasaki's procedure repeats the rotation check with a jog start right before the first real run.

For gear pumps, Parker gives a quick visual test in its PGP service literature: hold the pump with the shaft pointing at you and the idler gear below the shaft. On a clockwise pump, flow runs left to right, so the inlet is on the left. Turning the pump upside down swaps the ports but does not change its rotation.

Wrong rotation is not harmless. A piston pump turned backward is trying to pump out of its inlet, the rotating group is starved, and on many designs the shaft seal sees pressure. If the jog shows the wrong direction, stop and fix the wiring before anything else.

Why start a hydraulic pump unloaded, and how do you load it?

Fresh surfaces need to bed in under light load, and air compressed hard will diesel and erode parts. So every factory procedure starts at low pressure:

Continental (HPV manual): reduce the relief setting, start, bleed the air, cycle the unloaded machine at low pressure, then raise pressure gradually while checking the inlet lines for leaks. Only then increase speed, listening for changes in sound or air in the oil.
Parker (PV series): raise the pressure setting only once all the air is out, and run at reduced pressure for five to ten minutes while you check every connection.
Oilgear: do not start or stop the unit under load unless the system is approved for it, and do not run in neutral for long periods without a supercharge circuit.

On a pressure-compensated pump, "unloaded" means the compensator backed well off and the relief valve set above it. Once the system is purged, bring the compensator up to its working setting with a gauge on the outlet. Our guides cover the adjustment for Vickers compensators and the Rexroth A10VSO.

What should you watch during the first run?

Case drain flow and case pressure

You want oil coming out of the case drain, returning to tank at close to zero pressure. The start-up sheet for Vickers MHT vane motors in our files says flatly that some case flow must be seen before you keep running the motor. Case pressure limits on open-loop piston pumps are low: 0.5 bar on a Parker PV, 1 bar on a Kawasaki K3VL, 2 bar absolute on a Rexroth A10VSO, and under 25 psi on Oilgear's PVG and PVM. The full model-by-model table, with the factory case drain flow limits we found, is on our case drain flow test page.

Temperature

Continental caps temperature at the case drain at 180°F (82°C). Danfoss points out that the hottest spot in a hydrostatic loop is normally the motor case drain, so that is where to put the thermometer. A pump that heats up fast on its first run is leaking internally or working against a relief valve. See why is my hydraulic pump overheating.

Noise, foam and leaks

Watch for foam and milky oil, listen for the gravel sound of cavitation, and wipe fittings dry so a new weep shows. Whining and rattling at low pressure is usually an inlet problem, not a pump problem. See hydraulic pump making noise.

What do you check after the first hour and the first day?

Reservoir level. It will drop as cylinders, motors and lines fill. Danfoss tells you to keep the reservoir full throughout start-up, and Continental says to watch the level while you cycle every cylinder and run every motor at operating speed for 20 minutes.
Filter. At the end of that 20-minute cycle, Continental replaces the filter element and tops up with new, filtered oil. Start-up flushes out whatever the old failure left behind, and the first element catches it.
Extra filtration on new circuits. The MHT motor start-up sheet in our Vickers files recommends extra 5-micron filters on a new circuit wherever possible, at the tank return or ahead of the motor, and warns against thermal shock of more than 50°F on a new motor.
Re-check fittings and case drain. Re-torque anything that wept, read case pressure again with the oil at operating temperature, and record case drain flow as your baseline. That number is the one you will compare against for the rest of the pump's life.

How is start-up different for piston, vane, gear, hydrostatic and orbital units?

Everything above applies to open-loop piston pumps. The other types change the emphasis:

Vane pumps

Fixed vane pumps such as the Vickers V and VQ families rely on speed to push the vanes out against the ring, which is why Vickers quotes 600 rpm as the minimum drive speed for priming. Crack an outlet fitting if air is trapped. Variable-displacement vane pumps do have a case drain: Bosch Racine says VPV and SV shaft seals fail if case pressure goes over 10 psig (0.7 bar), and rules out spring-loaded check valves and shared drain headers in the drain line. Browse Vickers vane pumps or the V10/V20 catalog.

Gear pumps

Most gear pumps have no separate case to fill, so the job is getting oil into the gears before the first turn and getting the rotation right. Pour oil into the inlet port before you connect the suction line, and confirm whether the pump is single-direction or bi-rotational before you wire the motor. See Parker gear pumps.

Closed-loop hydrostatic: charge pressure first

In a closed loop the charge pump feeds the whole system, so nothing happens until charge pressure comes up. Danfoss' Series 90 start-up procedure runs like this:

Fill the pump housing through the upper case drain port and fill the inlet line from the reservoir.
Put a 50 bar (1,000 psi) gauge on the charge pressure port and disconnect the pump control signal so the pump stays in neutral.
With the engine disabled, crank for several seconds (within the engine maker's limit), wait 30 seconds, crank again, and refill the reservoir.
Once charge pressure reaches 3.5 bar (50 psi), start the engine and idle for at least 30 seconds while you check for leaks and listen for cavitation.
When charge pressure reaches the model code setting, bring the engine to operating speed, reconnect the control, and cycle forward and reverse for at least five minutes to purge the loop.

The older Sauer-Sundstrand Series 40 manual wants charge pressure at least 7 bar (100 psi) above case pressure. Eaton's heavy-duty hydrostatic guide gives 15.2 bar (220 psi) differential in neutral and 11.0 bar (160 psi) in forward or reverse, and warns that thick oil on a cold start can raise case pressure enough to blow motor shaft seals.

Read your own manual on cranking. Danfoss tells you to crank the engine to help fill the system. Denison's 1989 start-up bulletin for the P24P/M24G hydrostatic transmission (SP-514) says the opposite: do not turn the engine with the starter to prime, because its vane charge pump needs at least 700 rpm to pick up. Denison's procedure goes straight to 1,000 rpm, shuts down if replenish pressure has not started to rise within 15 seconds, and waits five minutes before trying again. Both are right for their own machines.

The same Denison bulletin pre-fills the cases, filters and loop lines, alarms on replenish pressure below 50 psi and case drain temperature above 180°F, runs 15 minutes at zero displacement, and expects case pressure to settle under 50 psi once hot. Hydrostatic parts and catalogs: Sundstrand, Eaton heavy-duty, Parker Gold Cup.

Orbital and vane motors

Char-Lynn orbital motors often run without a separate case drain, and then the shaft seal sees return line pressure. The Danfoss (formerly Eaton) Char-Lynn catalog gives standard shaft seal case pressure ratings of 100 bar (1,500 psi) for H, S and T motors, 140 bar (2,000 psi) for the 2000 Series, 100 bar for the 4000 Series, 70 bar (1,000 psi) for the 6000 Series and 20 bar (300 psi) for the 10,000 Series. If return pressure is higher than that, the motor needs its case drain connected. Eaton's R Series catalog adds a detail most people miss: if you do run a case drain, plumb it so the motor stays full, with a restriction that holds about 3.5 bar (50 psi) in the case. Brake motors need a separate drain straight to tank. See Char-Lynn orbital motors.

Vickers MHT vane motors have their own start-up sheet in our files: oil no thicker than 180 SSU at 100°F filled through a 5-micron filter, an hour of running to warm the oil to at least 120°F and purge air, then start the motor at 5 to 10 rpm loaded or stalled and confirm case flow. Increase speed in four or five steps about ten minutes apart, with the relief set at running pressure. A new MHT should not run above 50 rpm with less than 500 psi across it. More in the Vickers vane motor catalog.

Printable hydraulic pump start-up checklist

Print it and tick it off at the machine. Where your manual differs, the manual wins.

#	Step	Check	Done
1	Cause of failure found and fixed	Contamination, case pressure, wrong fluid, alignment	[ ]
2	System cleaned	Reservoir cleaned, lines flushed, new filter element	[ ]
3	Reservoir filled through a filter	Level allows for the oil the system will absorb	[ ]
4	Alignment and mounting	Coupling aligned, pump square on its pilot, no side load	[ ]
5	Case drain plumbed from highest port	Full size, own line to tank, ends below oil level	[ ]
6	Case filled	Filled through top drain port until it overflows	[ ]
7	Inlet ready	Suction valves open, line full, fittings tight	[ ]
8	Circuit unloaded	Relief and compensator backed off, or pump in neutral	[ ]
9	Gauges fitted	Outlet (or charge), case pressure, inlet vacuum	[ ]
10	Rotation confirmed	Jogged and matched to arrow or model code	[ ]
11	Primed and purged	Primes within seconds; oil clear, not milky	[ ]
12	Low-pressure run	Five to ten minutes unloaded; leaks checked	[ ]
13	Pressure raised in steps	Compensator and relief set with a gauge	[ ]
14	Case drain and temperature	Case flow visible, case pressure within limit, temperature steady	[ ]
15	After run-in	Filter element changed, reservoir topped up	[ ]
16	Baseline recorded	Case drain flow at known temperature, pressure and rpm	[ ]

Frequently asked questions

Do I need to fill a hydraulic pump case before starting it?

Yes, on any piston pump or motor with a case drain. The case oil lubricates the bearings, slippers and shaft seal, and the case only fills by internal leakage once the pump is running. Kawasaki warns that filling only the suction line is not enough and that seizure can follow very quickly with a dry case.

Which port do I use to fill the pump case?

The highest case drain port, with clean filtered oil, until it overflows. Parker, Rexroth, Danfoss and Oilgear all fill through the drain port, and all of them want the drain line taken from the highest port so the case stays full when the machine is shut down.

How do I prime a hydraulic pump that will not pick up oil?

Check that the suction valve is open, the reservoir is above the suction opening and every inlet fitting is tight. Then give the trapped air a way out by cracking an outlet fitting or running the circuit at free circulation. Vickers expects a vane pump driven at 600 rpm or more to prime almost immediately once the air can escape.

How long should a new pump run before I load it?

Until the air is out and the oil is clear, then a short run at low pressure. Parker calls for five to ten minutes at reduced pressure on the PV series, Vickers says five to ten minutes unloaded on the PVH, and Continental cycles every cylinder and motor for 20 minutes before changing the filter element.

Why is my new hydraulic pump not pumping?

The usual causes are wrong rotation, a closed suction valve, an air leak on the inlet, a low reservoir, or air trapped against a closed-center valve with nowhere to go. Vane pumps also need enough drive speed to prime. Check those before suspecting the pump.

How do I check hydraulic pump rotation?

Read the direction from the housing arrow or the model code, then bump the electric motor with the pump uncoupled and watch the coupling. Jog once more after coupling up. On a gear pump, Parker's test is to hold the shaft toward you with the idler gear below it: a clockwise pump moves oil left to right.

Should I crank the engine to prime a hydrostatic pump?

It depends on the unit. Danfoss' Series 90 procedure has you crank with the engine disabled to help fill the system, while Denison's P24P/M24G bulletin says never prime with the starter because its vane charge pump needs at least 700 rpm. Follow the start-up section of your own manual.

About the technical information on this page. Figures, procedures and part numbers here come from manufacturer service literature and are provided for reference only. They are only as accurate as the factory data, and the revision we have on file may not be the latest for your unit. Always confirm against the current manufacturer documentation for your exact model and follow safe work practices. RestoPower is not responsible for damage, injury or loss resulting from the use or misuse of this information, or from incorrect assembly, adjustment, installation or operation of any unit.

Rebuilt pump, or need one rebuilt?

Send us the model code and tell us how the last one failed. If it is coming to our shop, the teardown evaluation is free and you get a firm, itemized quote before any billable work, with a 1-year warranty on repair work. In-stock parts from the catalog carry a 2-year warranty and ship within 24-72 hours.

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