Why Is a Hydraulic System Losing Pressure?

A hydraulic system may appear to lose pressure because it is in a different control state, the measurement does not represent the working point, delivered flow is inadequate, or fluid is escaping through an unintended path. Restrictions can also leave adequate pressure at the pump while reducing pressure available to an actuator. Pressure falling after a stop is a separate observation that needs its own load, temperature, and valve-state context.

Start by defining where, when, and under what demand the pressure is low. Then compare the hydraulic supply, the relevant branch, and the actuator using suitable measurements taken under matching conditions. This article explains that diagnostic sequence and its limits. It does not prescribe relief settings, leakage acceptance values, or improvised isolation tests.

Contents

1. Define what losing pressure means
2. Verify the operating state and the measurement
3. Compare supply and actuator pressures
4. Check delivered flow at the relevant load
5. Trace control and possible leakage paths
6. Interpret pressure decay after stopping
7. Use temperature and timing to narrow the search
8. Confirm the cause and document the correction
9. Frequently asked questions

Define what losing pressure means

Write down the expected behavior before deciding that a fault exists. Low pressure while fluid circulates through an unloaded path can be intentional. A pressure-limited working stage, a load-sensing standby state, and a stopped isolated volume do not have the same expected pressure.

Use three categories to make the complaint specific:

Observation	Diagnostic question
Pressure is too low during required work	Is the required supply, control action, and load condition present?
Pressure is high upstream but low downstream	Where does pressure change while flow passes through the circuit?
Pressure decreases after stopping	What is happening to the isolated volume, load, temperature, and connected paths?

Also distinguish a measured pressure change from reduced force or slower movement. Those symptoms may be related, but they are not interchangeable. A machine can move more slowly at a similar pressure if usable flow has fallen.

Record the function, direction, load, speed command, fluid temperature, and whether other functions operate simultaneously. Note whether the change was sudden, gradual, or followed maintenance.

If the investigation concerns the power source alone, use power unit not building pressure. This guide follows the pressure question farther into the circuit and across different operating states.

Verify the operating state and the measurement

Read the current hydraulic diagram and control description. Identify the pump type, unloading arrangement, pressure controls, directional valves, pilot signals, and any pressure-reducing branch. A branch may intentionally operate below the main supply pressure.

Danfoss’s Series 45 control explanation, printed pages 14–17, distinguishes pressure-compensated and load-sensing behavior, including different standby conditions. The lesson is to compare the reading with the selected control’s intended state, not with a pressure remembered from another machine.

Confirm where the instrument connects and what pressure reference it uses. Check its range, units, calibration status, zero behavior when appropriate, and whether the display or logging method can resolve the event. A connection to a trapped pocket may not show the pressure in the active flow path.

For comparisons, use readings from the same operating moment or repeat a sufficiently controlled condition. Comparing a pump peak from one cycle with an actuator average from another can manufacture an apparent loss.

Do not connect instruments or disturb fittings without the approved installation and energy-control procedure. Pressurized fluid presents an injection hazard; HSE’s alert explains why leak investigation requires care. The hydraulic system working principles article helps locate supply, return, and working chambers.

Compare supply and actuator pressures

Pressure at a pump outlet does not directly state the force available at a cylinder. Flow through valves and lines requires pressure differences, and pressure in the opposite chamber also acts on the piston.

Consider an illustrative extension case. The cap-end area is 25 cm² and the rod-side annular area is 15 cm². Suppose readings taken at the relevant condition show 170 bar at the supply point, 110 bar at the cap end, and 20 bar at the rod end.

Ignoring friction and acceleration, net hydraulic extension force is:

F = p_cap A_cap − p_rod A_annular.

Because 1 bar acting on 1 cm² gives 10 N:

F = 10 × (110 × 25 − 20 × 15) = 24,500 N, or 24.5 kN.

The pressure-area relationship follows the basic definition described in OpenStax’s hydraulic principles. The example is an original hypothetical calculation.

If the required external axial force were 30 kN under these same idealized assumptions, the necessary cap-end pressure would be:

p_cap = (30,000/10 + 20 × 15)/25 = 132 bar.

The supply reading of 170 bar therefore does not by itself show that the cylinder can meet the requirement. This calculation is a comparison, not an instruction to increase a setting.

If 12 L/min passes from the 170 bar point to the 110 bar point through a dissipative flow path, its 60 bar drop corresponds to approximately 60 × 12/600 = 1.2 kW hydraulic power loss. Determine whether the drop is expected for the commanded function before calling it a fault. See cylinder bore and force calculations for the wider force model.

Check delivered flow at the relevant load

A pressure gauge is essential, but it does not measure delivered flow. A pump may reach a pressure condition while supplying less useful flow than the application requires.

Webtec’s maintenance explanation emphasizes the distinction between pressure and flow when diagnosing performance. Compare pump delivery at the relevant pressure, speed, and fluid condition with the applicable specification.

Review actual drive speed and available drive capability. Then consider inlet supply, fluid condition, the commanded displacement of a variable pump, and auxiliary demands. Do not treat every shortfall as internal pump wear.

A system-wide change can point toward a shared supply or control issue, whereas a single affected function can point toward its branch. These are ways to prioritize investigation, not rules that exclude all other possibilities. Shared valves and priority circuits can complicate the pattern.

Testing must use a planned arrangement that preserves pressure protection and respects the ratings and flow direction of all connected equipment. A casual blocked-outlet test can expose the assembly to unintended loads and does not reproduce every working condition.

The motor and pump sizing guide explains the relationship between delivered flow, speed, displacement, and shaft demand. Use those quantities together when deciding whether the supply is capable of the required operating point.

Trace control and possible leakage paths

Separate the command from the component’s actual response. An electrical command does not prove that a valve has shifted, that pilot pressure is available, or that the pump receives the intended demand signal.

Compare the circuit’s expected paths with the observed state. Possible investigation areas include a bypass remaining open, a pressure control responding differently from its specification, a restricted pilot or drain connection, or flow escaping through worn or damaged components.

Parker’s power unit maintenance guide lists supply, drive, valve, and leakage-related possibilities. Use such classifications to organize questions, while following current equipment-specific procedures for any test.

Possible path or issue	Evidence needed before assigning cause
External leakage	Located loss, affected circuit boundary, and condition of occurrence
Internal bypass	An approved flow or leakage assessment under stated conditions
Incorrect control state	Command, pilot conditions, actual response, and circuit requirements
Excessive restriction	Pressure difference and corresponding flow across the relevant path
Inadequate supply	Delivered flow, speed, inlet conditions, and pump control state

Internal leakage cannot be located merely by pointing to the hottest nearby surface. Temperature measurements may help, but heat can be transported and several losses can coexist.

Do not progressively cap lines or disconnect drains without a reviewed test plan. Such changes can defeat load control or trap pressure. For a cylinder-specific investigation, use cylinder internal leakage testing and retain its distinctions between the test boundary and the complete machine.

Interpret pressure decay after stopping

A falling pressure after the pump stops is not the same question as low pressure during flow. Identify the volume being observed and all paths still connected to it. A control valve may intentionally release pressure, while another part of the system remains charged.

In a nominally isolated volume, pressure can change with leakage, fluid temperature, compliance, trapped gas, or movement of a load or boundary. These influences can overlap. A gauge trace alone does not identify their individual contribution.

Record the starting pressure, temperature trend, valve state, elapsed time, actuator position, external load, and whether an accumulator is connected. Use the equipment’s specified stabilization and test conditions when a pressure-retention test is required.

Do not convert pressure decay into a leakage rate without an appropriate model and known test conditions. The effective volume and compressibility matter, as do temperature and any movement. There is no universal pressure-drop-per-minute value that proves a seal has failed.

A stationary gauge reading also does not prove that a suspended load is securely supported. Load-holding capability requires its own assessment and approved protection. See hydraulic cylinder drift for the relationship between movement, trapped pressure, and connected leakage paths.

Use temperature and timing to narrow the search

A complaint that appears only when warm helps define a repeatable condition, but it does not prove a worn pump. Temperature can change viscosity, leakage behavior, control response, and available cooling performance. The correct interpretation depends on the actual components and fluid.

Compare cold and warm records at matched speed, commanded function, and load. If those differ, the comparison may reflect a different operating point rather than a temperature effect.

Consider a hypothetical machine where one function becomes weak after sustained use, while other functions remain satisfactory. That pattern justifies checking the affected branch and its control conditions early. It does not completely exclude a shared supply problem.

Now suppose the pressure is normal upstream of the branch but falls across a particular flow path during the complaint. The next question is whether the pressure difference matches the intended valve state and flow. “Replace the pump” would skip that evidence.

A changed sound or visible fluid condition can add context; refer to noisy hydraulic power unit. Record those observations with the same time base as pressure data where practical. A useful trend connects events rather than collecting unrelated maximum readings.

Confirm the cause and document the correction

Choose the next test by the question it can resolve. Define the expected result if the hypothesis is correct, the result that would weaken it, and the operating limits that apply during the test.

The repair record should include the supported cause, work performed, replacement part identification, any approved control changes, and remaining uncertainties. Keep the original measurements so that improvement can be assessed rather than remembered.

After the correction, repeat the relevant condition and confirm both the required pressure behavior and the associated performance. Check whether delivered flow, actuator speed, temperature trend, and other functions remain acceptable. Restoring a gauge reading while reducing useful flow is not a complete recovery.

Preserve measurement location, instrument information, sampling details where relevant, load, speed, and fluid condition. The inconsistent test results guide explains why these details determine whether two records are comparable.

Do not close the investigation by increasing the relief or compensator setting to hide the symptom. That can change available force and component loading without correcting the cause. Any necessary setting revision belongs to an engineering review of pump, valve, and cylinder matching.

A completed investigation states what was confirmed and what was not. Where a required test could not be performed, identify the open condition explicitly instead of declaring the whole system restored.

Frequently asked questions

Is low pressure during standby always a fault?

No. Some control arrangements intentionally reduce pressure during standby or unloading. Compare with the circuit’s specified state.

Can pressure look normal while the machine moves slowly?

Yes. The pressure can still correspond to the load while usable flow is reduced. Measure or otherwise establish the relevant flow and operating conditions.

Does pressure loss when warm prove pump wear?

No. Wear is one possible cause, but fluid condition, branch losses, control behavior, and supply conditions must also be considered.

Does a pressure drop after shutdown prove a leaking piston seal?

No. Temperature, connected paths, compliance, load movement, and other leakage locations can affect the trace. Prepare the circuit revision, measurement locations, simultaneous operating data, and unresolved questions for a qualified engineering review.