Test case drain flow by putting a flow meter in the case drain line (or timing the flow into a measured container), with the oil at operating temperature and the pump on stroke at full working pressure, then again with the pump compensated. "Too much" is whatever the factory test figure for your unit says, and where we found one in a manufacturer's manual it is in the tables below; where no figure exists, compare the reading against a baseline taken when the unit was new or freshly rebuilt.
That last part matters, because a universal percentage is a rule of thumb, not a specification. The factories that do publish limits give them model by model, at a stated speed, pressure and temperature, and they differ a lot.
Factory figures beat rules of thumb. Of the manuals we checked, only Continental gives a percentage rule (drain flow below 5% of discharge at rated pressure on its HPV pumps). Vickers, Parker and Denison give actual gallons or liters per minute for each size. Where your unit is listed below, use that number. Where it is not, use your own baseline.
What is case drain flow?
A piston pump or motor is built with running clearances between the pistons and their bores, the cylinder block and the valve plate, and the slippers and the swashplate. Oil leaks across those clearances on purpose. Danfoss describes it in its Series 90 manual as the small flow from the block-to-valve-plate and slipper-to-swashplate interfaces that lubricates and cools them, which the case drain ports carry back to the reservoir.
As those surfaces wear, the gaps open up, more oil goes to the case instead of the outlet, and three things happen together: delivery drops, the oil runs hotter, and case drain flow rises. That is why the drain line is the one place you can read wear directly without taking the unit apart.
Why is case drain flow the best single health test for piston pumps and motors?
- It measures the wear itself. Pressure gauges tell you the system is weak. Case drain flow tells you whether the rotating group is the reason.
- It can be done in place. One flow meter in a low-pressure line, no teardown.
- It trends. One reading a month at the same conditions shows a pump wearing out long before it stops working.
- It separates pump from motor. In a hydrostatic loop, Danfoss tells you to check the motor by monitoring its case flow with loop flushing defeated, and to replace the motor if the flow is excessive.
Continental's HPV troubleshooting table makes the heat connection directly: if the fluid is overheating because of excessive pump leakage, recheck case drain flow and repair as required. See also why is my hydraulic pump overheating.
How do you measure case drain flow?
- Get the oil to operating temperature. Leakage rises as oil thins out, so a cold reading flatters a worn pump. The factory tests below are all run at about 120°F: Vickers' service center procedures at 120°F ±10°F, Parker's P1/PD test at 110-130°F, and Denison's PV/PVT test at 120°F ±10°F.
- Install the meter in the drain line. Use a low-restriction flow meter rated for the drain flow you expect, with a 0-100 psi (or 0-10 bar) gauge teed in at the case. The meter must not push case pressure past the unit's limit; Vickers keeps case pressure to 5 psi or less when testing PVB and PVQ pumps, and 0-5 psi on the PVH.
- Measure at full stroke and full pressure. Load the pump to its rated or working pressure while it is still on stroke (flow going to a load valve or the circuit), and read the drain flow. Note rpm and temperature at the same time.
- Measure again compensated. Deadhead the pump against its compensator and read the drain flow again. This number is always higher than the full-stroke reading, by design.
- Compare. Against the factory figure if your unit is listed, or against your baseline from the same speed, pressure and temperature.
The timed container method
No flow meter? Run the drain hose into a clean, measured container for a timed period, then work it out:
gpm = gallons collected ÷ minutes timed. One US gallon is 231 cubic inches or about 3.785 liters, so a quart (0.25 gal) collected in 30 seconds (0.5 minute) is 0.5 gpm.
Keep it short, top the reservoir back up straight away, and keep the hose end submerged in the container so you are measuring oil, not splash. Hot oil burns: wear gloves and a face shield.
Never cap, plug or valve off a running pump's case drain to "see what happens." Case pressure rises immediately and the shaft seal is the weakest point. Factory test stands do block the drain briefly on some Vickers pumps, but only to check a built-in case-to-inlet check valve, with gauges on, under controlled conditions.
Case drain flow formula
To express the reading as a percentage, divide by the pump's rated or theoretical flow:
- Case drain % = case drain flow ÷ pump flow × 100
- Theoretical flow (gpm) = displacement (cu in/rev) × rpm ÷ 231
Worked example with Continental's own numbers: an HPV20 is rated at 20 gpm at 1,750 rpm, so its 5% line at rated pressure is 1.0 gpm of drain flow. Remember the percentage only means something for the brand that published it.
What test conditions change the reading?
| Condition | Effect on case drain flow |
|---|---|
| Oil temperature | Hotter, thinner oil leaks more. Test hot, and always at the same temperature. |
| Pressure | Leakage rises with pressure. A reading at 500 psi says little about the pump at 3,000 psi. |
| Speed | Factory limits are tied to a test rpm. Vickers' PVE figures are 1.45 gpm at 1,200 rpm in the overhaul manual and 1.70 gpm in the 2,400 rpm service center test. |
| Full stroke vs. compensated | Compensated (deadheaded) readings are higher because the control itself bleeds to case. |
| Control type | Denison adds 0.8 gpm for a C compensator and 0.9 gpm for F, L, J, K and T controls on the PV/PVT. Parker adds 2.3 L/min to compensated leakage for its A series compensators on the P1/PD. |
| Case pressure | Back pressure on the drain changes the reading and can damage the shaft seal. Keep it inside the limits below. |
Allowable case drain flow, model by model, from factory manuals
These are the figures we could source from manufacturer documents on file. Each limit is a maximum at the stated test condition unless noted. If your model is not here, we did not find a published figure, and we would rather say so than invent one.
Vickers PVB and PVQ (service center test procedure, June 1997)
Fluid at 120°F ±10°F, case pressure 5 psi or less, pump at full stroke. "Cutoff" is the pump compensated at the same pressure.
| Model | Test speed | Test pressure | Max drain at full stroke | Max leakage at cutoff |
|---|---|---|---|---|
| PVB5 | 1,200 rpm | 3,000 psi | 0.35 gpm (81 cu in/min) | 0.50 gpm |
| PVB6 | 1,200 rpm | 2,000 psi | 0.25 gpm (58 cu in/min) | 0.40 gpm |
| PVB10 | 1,200 rpm | 3,000 psi | 0.75 gpm (173 cu in/min) | 1.20 gpm |
| PVB15 | 1,200 rpm | 3,000 psi | 0.55 gpm (127 cu in/min) | 1.00 gpm |
| PVB20 | 1,200 rpm | 3,000 psi | 1.15 gpm (266 cu in/min) | 2.00 gpm |
| PVB29 | 1,200 rpm | 2,000 psi | 0.85 gpm (196 cu in/min) | 1.60 gpm |
| PVQ10 | 2,400 rpm | 3,000 psi | 0.50 gpm | 1.10 gpm |
| PVQ13 | 2,400 rpm | 2,000 psi | 0.47 gpm | 0.95 gpm |
| PVQ20 | 2,400 rpm | 3,000 psi | 1.00 gpm | 1.50 gpm |
| PVQ32 | 2,400 rpm | 2,000 psi | 0.75 gpm | 1.25 gpm |
The same procedure sets a minimum too: at 100 psi the PVB drain must show at least 0.6 cu in/min, so a pump with no drain flow at all is also a reject. Catalogs and kits: PVB, PVQ.
Vickers PVE (overhaul manual M-2854-S)
| Model | Max case leakage at 100 psi | Max case leakage at system pressure | Max at cutoff |
|---|---|---|---|
| PVE12 | 620 cc/min | 1.12 gpm at 3,000 psi | 2.0 gpm |
| PVE19 | 800 cc/min | 1.45 gpm at 3,000 psi | 2.0 gpm |
| PVE21 | 800 cc/min | 1.45 gpm at 2,700 psi | 2.0 gpm |
Test at 1,200 rpm, 120°F ±5°F, full stroke, case pressure no more than 15 psi. Vickers also requires at least 10 cc/min of case leakage at 100 psi. The 1997 service center procedure tests the PVE19 and PVE21 at 2,400 rpm instead and allows 1.70 gpm at system pressure and 2.25 gpm at cutoff. More in the PVE catalog.
Denison PV/PVT (service information S1-AM009-L)
| Model | Max rated continuous pressure | Max case leakage at rated pressure, full flow |
|---|---|---|
| PV/PVT6 | 3,500 psi | 0.53 gpm (2.0 L/min) |
| PV/PVT10 | 3,500 psi | 0.58 gpm (2.2 L/min) |
| PV/PVT15 | 3,500 psi | 0.90 gpm (3.4 L/min) |
| PV/PVT20 | 3,500 psi | 1.2 gpm (4.5 L/min) |
| PV/PVT29 | 3,000 psi | 1.5 gpm (5.7 L/min) |
Test at 1,770 rpm ±30, 120°F ±10°F, case pressure 3-10 psi. When compensated, Denison allows a further 0.8 gpm (3 L/min) for the C compensator and 0.9 gpm (3.4 L/min) for F, L, J, K and T controls on top of the full-flow figure. See Denison piston pumps.
Denison P6, P7, P8 Gold Cup (service publication LT3-00058-2)
| Displacement | Max case drain at 1,000 psi | Max case drain at 5,000 psi |
|---|---|---|
| 6.0 cu in (P6) | 1.5 gpm (5.7 L/min) | 3.0 gpm (11.4 L/min) |
| 7.25 cu in (P7) | 1.8 gpm (6.8 L/min) | 3.6 gpm (13.6 L/min) |
| 8.0 cu in (P8) | 1.5 gpm (5.7 L/min) | 3.0 gpm (11.4 L/min) |
Full volume, read above and below center. Note the test conditions: 130°F ±10°F and a deliberate case pressure of 65 psi ±10, which the Gold Cup is built to take. See Parker Gold Cup pumps.
Parker P1/PD (maintenance bulletin HY28-2665-02/SVC/EN)
| Size | Case leakage at 280 bar, full stroke | Case leakage compensated at 280 bar |
|---|---|---|
| 018 | 1.6 L/min | 5.7 L/min |
| 028 | 2.3 L/min | 5.7 L/min |
| 045 | 5.3 L/min | 9.5 L/min |
| 060 | see note | 11 L/min max |
| 075 | see note | 15.1 L/min max |
| 100 | see note | 15.9 L/min max |
| 140 | see note | 16.7 L/min max |
Test at 1,800 rpm, 43-54°C (110-130°F), case pressure no more than 1 bar (14.5 psi). Add 2.3 L/min to the compensated figure with A series compensators. Parker's table prints the full-stroke figure for the 060 to 140 sizes as a minimum rather than a maximum, so we have left those out rather than guess what was meant. See Parker P1 pumps.
Vickers MHT vane motors (service center test procedure, June 1997)
Motors need case flow too, and here the factory sets a minimum as well as a maximum. Readings at 2,000 psi differential, oil at 120°F ±5°F at the motor inlet.
| MHT series | Test speed | Case flow min | Case flow max |
|---|---|---|---|
| MHT32 | 150 rpm | 0.10 gpm | 0.6 gpm |
| MHT50 | 100 rpm | 0.25 gpm | 1.0 gpm |
| MHT70, MHT90 | 100 rpm | 0.40 gpm | 1.2 gpm |
| MHT110, MHT130, MHT150 | 100 rpm | 0.40 gpm | 3.0 gpm |
| MHT190, MHT220 | 50 rpm | 0.50 gpm | 2.0 gpm |
| MHT250 | 50 rpm | 0.50 gpm | 2.25 gpm |
| MHT300 | 100 rpm | 0.80 gpm | 4.5 gpm |
| MHT380, MHT410, MHT440, MHT500 | 50 rpm | 0.80 gpm | 4.0 gpm |
| MHT690, MHT750 | 50 rpm | 1.0 gpm | 5.0 gpm |
| MHT1000 | 50 rpm | 1.2 gpm | 6.0 gpm |
Vickers also warns not to run a new MHT above 50 rpm with less than 500 psi across it, because low pressure may not provide enough lubricating case flow. More in the Vickers vane motor catalog.
Continental HPV
Continental's HPV Installation and Start-Up manual (R90.00) gives the percentage rule: drain flow below 5% of discharge at rated pressure.
Units with no published drain flow figure
Vickers grades the PVH on delivery loss at rated pressure rather than a case drain number. In the Rexroth, Danfoss, Oilgear, Kawasaki, Eaton heavy-duty and Char-Lynn literature we checked, we found case pressure limits but no allowable case drain flow. For those, your baseline is the measuring stick, or ask us and we will tell you what we have.
What should case drain pressure be?
Low. On most open-loop piston pumps the shaft seal is a lip seal with case pressure on one side and the atmosphere on the other, and the limits are a few psi. Closed-loop units and Gold Cups are built for more. These are the factory limits we found:
| Unit | Case pressure limit | Source |
|---|---|---|
| Parker PV (design 44+) | 0.5 bar (2 bar peak), also when compensating | HY30-3245-INST |
| Parker P1/PD | Rated 1.0 bar gauge, peak 3.0 bar gauge, and under 0.5 bar above inlet | HY28-2665-02/SVC |
| Parker/Denison Gold Cup P6-P30, M series | 75 psi continuous, 125 psi intermittent; open circuit max 25 psi above inlet | HY28-2667-01/GC |
| Denison PV/PVT | 10 psi continuous, 15 psi intermittent; never more than 15 psi above inlet | S1-AM009-L |
| Continental HPV | 10 psi; never more than 15 psi above inlet | R90.00 |
| Rexroth A10VSO 31 and 32, A10VO 5x | 2 bar absolute, and max 0.5 bar above inlet at port S | RE 92711, 92714, 92703 |
| Rexroth A4VSO | About 4 bar absolute; the allowable value depends on drive speed | RE 92050 |
| Rexroth A4VG series 40 | Peaks to 10 bar (145 psi) under 0.1 s; continuous limit from a speed chart | RE-A 92004 |
| Kawasaki K3VL | 1 bar continuous, 4 bar peak | K3VL technical brochure |
| Oilgear PVG, PVM, XD5 | Under 25 psi (1.7 bar); higher needs a special shaft seal | 947023, 947070-B, 910003 |
| Danfoss Series 90 | 3.0 bar (40 psi) rated, 5.0 bar (75 psi) max at cold start | 520L0818 |
| Danfoss Series 51 motors | 3 bar (44 psi) rated, 5 bar (73 psi) cold start | 11008567 |
| Danfoss Series 45 (E and H frame) | 0.5 bar (7 psi) above inlet continuous, 2 bar (29 psi) cold start | 520L0606, 520L0611 |
| Sauer-Sundstrand Series 40 M25 | 1.7 bar (25 psi) continuous, 5.2 bar (75 psi) cold start | BLN-10056 |
| Eaton heavy-duty hydrostatics | Under 2.8 bar (40 psi) for any extended period | E-PUPI-TM009-E |
| Bosch Racine VPV and SV vane | 10 psig (0.7 bar) | 9535233783 |
| Linde HPV-02 | Not above 2.5 bar absolute in operation | HPV02 service and start-up |
| Char-Lynn orbital motors, standard seal | 100 bar H, S, T and 4000; 140 bar 2000; 70 bar 6000; 20 bar 10,000 Series | BC453983407117en |
Note the difference between "bar absolute" (Rexroth, Linde) and gauge readings. 2 bar absolute is only about 1 bar on a normal gauge at sea level.
Why does case pressure blow shaft seals?
Because the seal lip is only held against the shaft by its own preload. Rexroth's A4VG data sheet spells out what shortens shaft seal life: more frequent pressure peaks, higher average case-to-atmosphere differential, and higher speed. Danfoss warns that case pressure over the stated limits can damage seals, gaskets and housings and cause external leakage. Eaton adds that thick oil on a cold start can raise case pressure enough to blow motor shaft seals.
The usual plumbing culprits:
- A drain line that is too small, too long, or full of elbows. Parker limits the PV drain to 2 m before you go up a pipe size.
- Several drains teed into one header. Bosch Racine forbids it on the VPV and SV, and Rexroth says a shared drain must be sized so no unit exceeds its limit, especially at cold start.
- A spring-loaded check valve in the drain line. Racine rules them out on the VPV and SV, and Rexroth allows a drain check valve on the A10VSO only case by case with its approval.
- The drain plumbed into a pressurized return line instead of straight to tank below the oil level.
If the shaft seal keeps leaking, read hydraulic pump leaking from the shaft before you fit another one.
How do you read the result?
- Take a baseline right after start-up or rebuild, hot, at working pressure, with rpm noted. Write it on a tag on the pump.
- Compare like with like. Same temperature, same pressure, same speed, same stroke condition. A reading taken differently is not a trend.
- Watch the slope. A slow climb over months is normal wear and gives you time to plan a rebuild. A jump between two readings means something let go.
- Too little is a fault too. Vickers rejects a PVB with almost no drain flow and requires visible case flow on MHT motors. Zero flow on a running pump can mean a blocked drain, which is about to become a blown seal.
- Do not condemn a pump on the compensated reading alone. Every factory table above allows more leakage at cutoff than on stroke.
What does high case drain flow mean?
High drain flow means oil is getting past a worn sealing surface. The usual places:
- Valve plate and cylinder block face. Scoring between the kidney ports is the classic sign. Vickers' service center rules allow light scoring to be lapped out and scrap plates with deep cuts between the ports.
- Pistons and bores. Scratched or worn bores, or piston shoes with roll marks. Vickers notes that a rolled shoe changes the balance of the shoe area and creates lift.
- Slippers and swashplate. Heavy wear on the swashplate face or worn-through surface treatment.
- The control. A worn compensator spool, or the wrong orifice. Oilgear's PVM bulletin lists an oversized or missing OP2 stability orifice as a cause of excessive case drain, and says that failing to port a remote pressure control's exhaust to the HP port gives significantly higher case drain leakage.
Oilgear's troubleshooting chart also lists a worn or grooved cylinder barrel and port plate under excessive heating, which is the same wear showing up as heat. When the drain flow is over the limit, the repair is the rotating group, and usually the whole group: Vickers recommends replacing it as a set rather than piece by piece.
Repair or replace?
Catch it early and it is a rotating group and seal kit. Leave it until the pump stops making pressure and you are often buying a housing or shaft as well. Our repair cost guide explains where the money goes, and hydraulic pump not building pressure covers the other causes of weak output.
Frequently asked questions
How much case drain flow is normal?
It depends on the model, speed, pressure and temperature. Vickers, for example, allows a PVB10 a maximum of 0.75 gpm at 3,000 psi and 1,200 rpm, while Denison allows a PV29 1.5 gpm at 3,000 psi and 1,770 rpm. Use the factory figure for your unit, or your own baseline taken when the pump was new or rebuilt.
What is the case drain flow formula?
Case drain percentage equals case drain flow divided by pump flow, times 100. Theoretical pump flow in gpm is displacement in cubic inches per revolution times rpm, divided by 231. Continental uses a 5% limit at rated pressure for its HPV pumps, but other makers publish flows per model instead.
What should hydraulic pump case drain pressure be?
Low on most open-loop piston pumps: Parker's PV allows 0.5 bar, Denison's PV/PVT and Continental's HPV allow 10 psi, and Oilgear's PVG and PVM need under 25 psi. Closed-loop units are higher, for example Danfoss Series 90 at 3 bar rated. Check your unit's limit and keep the drain line short, full size and on its own to tank.
What causes high case drain flow?
Wear on the internal sealing surfaces: the valve plate and cylinder block face, the piston bores, the slippers and swashplate, or a worn or mis-orificed control. High drain flow usually comes with lower delivery and hotter oil.
Can I measure case drain flow with a bucket?
Yes. Run the drain hose into a measured container for a timed period and divide gallons by minutes. A quart in 30 seconds is 0.5 gpm. Test hot and at working pressure, keep it short, top up the reservoir, and protect yourself from hot oil.
Should I test at full pressure or deadheaded?
Both. Factory procedures read case drain on stroke at rated pressure and again with the pump compensated. The compensated figure is always higher because the control bleeds to case, so compare each reading against its own limit.
Do hydraulic motors have case drain flow too?
Yes, and some have a minimum as well as a maximum. Vickers' MHT test procedure, for example, requires between 0.40 and 1.2 gpm of case flow on an MHT70 at 2,000 psi. In a hydrostatic loop, Danfoss checks the motor by measuring its case flow with loop flushing defeated.
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.
Drain flow over the limit?
Send us the model code, the reading and the conditions you took it at. If the unit comes in, the teardown evaluation is free and you get a firm, itemized quote before any billable work, with a 1-year warranty on repair work. Parts bought from our catalog carry a 2-year warranty on in-stock parts, and in-stock parts ship within 24-72 hours.
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Related reading
- Hydraulic pump start-up procedure after a rebuild
- Hydraulic pump leaking from the shaft
- Hydraulic pump not building pressure
- Why is my hydraulic pump overheating?
- Hydraulic calculator
- Brand catalogs: Vickers, Denison, Parker Gold Cup, Rexroth, Sundstrand, Oilgear, Kawasaki, Eaton heavy-duty
- Seal kits: Vickers piston, Denison, Parker, Rexroth, Sundstrand
- Char-Lynn Orbital Motor Troubleshooting: Not Turning, Slow, Weak or Leaking
- Hydrostatic Transmission Troubleshooting
- Rexroth A10VSO and A10VO Pump Troubleshooting