How to Bleed Air from a Hydraulic Cylinder

by | Jul 23, 2026 | single acting cylinder

Bleeding air from a hydraulic cylinder is a straightforward procedure when the correct technique is used — but skipping it after installation or maintenance produces a system that performs poorly from the first operation and may damage seals within hours. Air trapped in a hydraulic cylinder circuit is compressible, unlike hydraulic oil, and its presence makes the cylinder feel spongy, move in jerks, and generate pressure spikes at end of stroke. This guide covers the complete bleeding procedure for single acting and double acting cylinders.

HCYY11112016 grain tank lifting cylinder bleed air procedure after installation

Why Bleeding Is Necessary

Hydraulic oil is essentially incompressible — its bulk modulus is approximately 1,500–2,000 MPa. Air is highly compressible — at atmospheric pressure, 1 litre of air compresses to less than 1 ml at 200 bar. This difference in compressibility is why even a small volume of air trapped in a hydraulic cylinder circuit causes dramatically different behaviour. When the directional valve opens, the pump must first compress the trapped air before pressure builds to move the load. This compression phase produces a delay followed by a sudden lurch as the air reaches the pressure needed to move the load — the classic aeration judder.

After any maintenance that involved disconnecting a hydraulic line, installing a new cylinder, or draining and refilling the reservoir, the circuit contains air at whatever volume the disconnected components held. Bleeding the air out before full-load operation protects seals from the pressure spikes caused by air collapse and ensures the cylinder performs to its rated response time and accuracy.

Bleeding Procedure for Single Acting Hydraulic Cylinders

1
Set system pressure to 30–50% of working pressure

Reduce the relief valve setting or use a pressure-reducing valve in the test circuit. Low initial pressure prevents hydraulic hammer if a large air pocket collapses suddenly during the first cycles.

2
Cycle slowly through full stroke 8–10 times with no load

Remove the load from the cylinder before bleeding. Extend fully, pause 2 seconds, retract fully, pause 2 seconds. Repeat 8–10 times. Each cycle pushes trapped air toward the reservoir through the directional valve return port. Slow cycling (below 50 mm/s) keeps air moving as bubbles rather than re-entraining it as foam.

3
Open bleed screw if fitted — during extension only

Many cylinders have a bleed screw at the highest point of the cap-end chamber. Crack it open by one turn while extending slowly. Allow oil to flow until no bubbles are visible in the oil stream, then close the screw before the next retraction. Never open the bleed screw during retraction — it will draw air in rather than bleed it out.

4
Return to full working pressure and cycle twice more

Raise the system pressure to full working pressure. Cycle the cylinder twice through the full stroke. The final traces of dissolved air are purged at full operating conditions.

5
Check reservoir oil level and top up

Oil filling the previously empty cylinder volume comes from the reservoir. Check the reservoir sight glass and top up to the correct level with the correct oil grade to prevent the level dropping low enough to cause suction-side air ingress during future operation.

Ever-Power hydraulic cylinder system reservoir bleed procedure oil level check

Bleeding Double Acting Hydraulic Cylinders

Double acting cylinders have air in both the cap-end and rod-end chambers after maintenance. The bleeding procedure cycles air out of both chambers alternately. Use the same low-pressure slow-cycling approach: extend fully, pause, retract fully, pause, repeat 8–10 times. If the cylinder has bleed screws at both ends, open the cap-end bleed during extension (bleeding cap-end air) and the rod-end bleed during retraction (bleeding rod-end air). Both screws must be closed before reversing direction.

Persistent Aeration: Finding the Air Ingress Source

Symptom Likely Air Ingress Source Fix
Oil milky or foamy in reservoir Suction line leak or low oil level drawing air Check oil level; inspect all suction line fittings
Aeration returns within minutes of bleeding Active suction-side air leak Pressurise suction line with inert gas to locate leak
Aeration only after high-speed extension Cavitation — pump inlet restriction Clean suction filter; check suction line bore is adequate
Aeration only after maintenance Air introduced during line reconnection not fully bled Repeat bleeding procedure with more cycles at lower speed
Aeration worse when oil is cold Dissolved air released as viscosity drops on warming Allow longer warm-up time; check oil grade is correct for ambient temperature

For hydraulic systems that repeatedly develop aeration despite correct bleeding procedures, the suction side of the hydraulic circuit must be inspected with the system running. Brushing liquid around suction line fittings while the pump is operating reveals air leaks by foaming at the leak point. Contact Ever-Power for guidance on suction circuit design and bleed port specification on new cylinders.

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Frequently Asked Questions

1. How to bleed air from a hydraulic cylinder?+
To bleed air from a hydraulic cylinder: set system pressure to 30–50% of rated working pressure, cycle the cylinder slowly through its full stroke 8–10 times with no load, operate with the bleed port (if fitted) at the highest point cracked open during each extension stroke, then bring to full working pressure and cycle 2–3 more times. Check the reservoir oil level after bleeding — filling the cylinder with oil reduces the reservoir level. Persistent aeration after this procedure indicates an ongoing air ingress point that must be found and sealed.
2. Why is there air in my hydraulic cylinder?+
Air enters a hydraulic cylinder system from four sources: a low reservoir oil level that allows the suction line to draw air into the pump; a leak in the suction line (on the atmospheric-pressure side of the pump) that allows air to be drawn in with the oil; air released from dissolved state when oil pressure drops below the saturation point (cavitation); and air introduced during maintenance when hydraulic lines are disconnected and reconnected without proper bleeding. Air in the reservoir often appears as milky or foamy oil.
3. What happens if you don’t bleed air from a hydraulic cylinder?+
Unbled air in a hydraulic cylinder causes: jerky, inconsistent extension because compressed air in the supply line must be compressed before pressure builds to move the load; spongy, unpredictable position control because the air acts as a compressible spring in the circuit; hydraulic hammer at end of stroke as air pockets collapse suddenly at high pressure; accelerated seal wear from pressure spikes associated with air collapse; and progressive damage to pump components from cavitation if the air originates in suction-side ingress.
4. How long does it take to bleed air from a hydraulic cylinder?+
For a standard single acting cylinder that has been correctly bled: 5–10 minutes using the slow-cycling procedure at low pressure. For a large double acting cylinder with long lines: 15–20 minutes. For a system that has been fully drained and refilled (such as after a major component replacement): 20–30 minutes of cycling plus checking. Persistent aeration that does not resolve after 30 minutes of cycling indicates an ongoing air ingress source that must be sealed before bleeding will be effective.
5. Does a single acting hydraulic cylinder need bleeding?+
Yes. A single acting hydraulic cylinder needs bleeding after: initial installation (new lines and fittings contain air); any maintenance that involved disconnecting the cap-end line; replacing the hydraulic pump or reservoir; and whenever the system displays symptoms of aeration — jerky extension, spongy feel, or noisy operation. The single acting cylinder’s single-line circuit simplifies bleeding because air can only enter and exit through the cap-end port.