How to Read a Hydraulic Test Report: What the Numbers Actually Mean

A remanufactured pump comes back with a test report attached. For some distributors, that document is a formality. For others, it's a critical quality signal.

The difference comes down to knowing what the numbers are actually measuring and what acceptable performance looks like at each test point.

What a Hydraulic Test Report Is Actually Verifying

A test report documents pump performance under controlled, loaded conditions on a test stand. It is not a visual inspection summary or a parts checklist. The data captures how the unit behaves when fluid is moving, pressure is applied, and the pump is asked to do the job it was built to do.

Most test reports record performance at multiple operating points, typically including no-load speed, rated pressure, and sometimes elevated pressure to confirm margin. Each data point reflects a specific condition.

Comparing results across those conditions tells you whether the pump maintains efficiency as load increases, which is exactly what determines real-world reliability.

Pump Condition: Flow Rate and Volumetric Efficiency

Flow rate is the most direct indicator of pump condition. At a given speed and pressure, a pump in proper condition should deliver flow within a defined tolerance of its rated output. That tolerance is typically stated as a percentage of theoretical displacement.

Volumetric efficiency is the ratio that matters most. It compares actual output flow to theoretical flow and accounts for internal leakage past the piston and valve plate interface. A piston pump operating below 90 to 92 percent volumetric efficiency at rated pressure is losing more fluid internally than acceptable tolerances allow. Bypass losses at that level generate heat, reduce actuator response, and accelerate downstream wear.

On a Rexroth A10V or Vickers PVB series unit, a test report should show volumetric efficiency at or above OE specification for the displacement and speed being tested.

Piston Pumps: Case Drain Flow & Internal Leakage Tolerances

Case drain flow is a secondary metric, but it belongs on every test report for piston pumps. It represents the volume of fluid that escapes past the rotating group and collects in the pump housing before returning to the tank. In some cases, drain flow is normal and necessary for lubrication. Elevated case drain flow signals worn slipper pads, a scored barrel bore, or a degraded valve plate. 

Watch for case drain measurements that exceed the OEM specification by more than 10 to 15 percent. At that threshold, the hydrodynamic film supporting the rotating group is compromised. 

The pump may test within acceptable flow output under short-duration stand conditions but fail early in service under sustained load. A test report without case drain data is an incomplete document.

Pressure Stability: What It Says About Valve & Control Condition

Pressure data on a test report covers more than peak capability. Stability across the test cycle is equally important. A pump that reaches rated pressure but shows oscillation or drop-off during the hold period may have a worn pressure compensator, a faulty control piston, or contamination affecting the servo response.

On variable displacement units like Denison Gold Cup piston pumps, the compensator setting and the actual regulated pressure need to align within a tight band. If the test report shows the compensator engaging at a pressure significantly below the set point, or failing to hold pressure steady across the test duration, that is a control-side issue, not a rotating group issue. Understanding that distinction helps distributors communicate accurately with their customers about what was found and what was corrected.

Using Test Documentation as a Distributor Tool

A detailed hydraulic test report does more than confirm that a pump is functional. It gives distributors documented proof of performance to stand behind when reselling a remanufactured unit.

It shortens conversations about warranty.

It gives technical staff a baseline for troubleshooting if a unit comes back with a field complaint.

A supplier that provides complete test documentation, with actual measured values rather than pass/fail checkmarks, is giving distributors a tool they can use.

Hydraulic Parts Source has delivered OE-quality remanufactured pumps and valves for 30 years. Contact us at sales@hydparts.com or call (866) 826-4122 to discuss your next repair or remanufacturing need.