Key Takeaways

• A pressure test tells you whether the system can reach its relief pressure — it does not by itself measure flow.
• Every reading must be taken with the hydraulic oil at normal operating temperature; cold oil hides faults and gives false good news.
• Low main relief pressure that drops further when hot = internal leakage, usually pump wear or a faulty relief valve.
• Pressure that is high or slow to relieve = a stuck or mis-set relief valve, not a weak pump.
• In a tandem-pump machine, test each pump separately to isolate a single-pump fault.
• When pressure is correct but the machine is slow, the fault is flow — confirm with a flow test before replacing the pump.

Table of Contents

• 1. What a pressure test tells you — and what it does not
• 2. Safety and preparation
• 3. Step-by-step: the main relief pressure test
• 4. Reading the results: what the numbers mean
• 5. The three pressures worth measuring
• 6. When pressure is fine but the machine is slow: add a flow test
• 7. Common mistakes that give false readings
• 8. Buying and service information
• 9. Frequently asked questions

1. What a pressure test tells you — and what it does not

A pressure test is the single most useful field diagnosis for an excavator hydraulic problem, because it is quick, safe when done properly, and it separates two very different faults. The system pressure is limited by the main relief valve; if the pump cannot build that pressure, it is leaking internally. If it builds too much, the relief is at fault. Either way, one gauge reading narrows the problem to the pump or the relief valve.

What a pressure test does not do is measure flow. A pump can make full pressure at a stalled function and still deliver far too little flow to move the machine at working speed. That is why a slow machine with a normal pressure reading needs a flow test next — a distinction that saves people from replacing a serviceable pump. The pumps we cover, from Hitachi main pumps to excavator main pumps across brands, are all diagnosed the same way.

2. Safety and preparation

• Park safely. Level ground, boom and bucket lowered, engine off, and the hydraulic system depressurised before you connect anything.
• Use a gauge rated well above system pressure and a hose in good condition. A burst test hose at 30 MPa is a serious hazard.
• Clean before you open anything. Dirt introduced at the test port is a future pump failure.
• Know the specification. Have the machine's service data to hand for the main relief pressure, the pilot pressure and the oil temperature range.
• Work with a helper or a safe remote procedure — you should never be reaching into the machine while an operator is stalling a function.

3. Step-by-step: the main relief pressure test

1. 1. Warm the machine. Run it until the hydraulic oil reaches normal operating temperature. Every figure below assumes warm oil.
2. 2. Connect the gauge. Fit the calibrated pressure gauge to the main pump pressure test port, using the correct adapter for the machine.
3. 3. Stall a function against relief. Following the machine's service procedure, operate a function to the end of its travel (or use the specified stall method) so the system relieves. Hold it only as long as the procedure states.
4. 4. Read and record. Note the highest steady pressure, and note the oil temperature at the same time — the two together are the meaningful result.
5. 5. Repeat at temperature. Take a second reading after the oil is fully hot; a pressure that falls noticeably hot is the fingerprint of internal leakage.
6. 6. Test each pump of a pair. If the machine has two main pumps, isolate and test each so a single-pump fault is not masked by the healthy one.
7. 7. Compare with specification. Judge the reading against the machine's service data, not against a general rule.

For reference on the parts side, the same test applies whether you are checking a Hitachi HPV145 pump or a ZX200 HPV102 main pump.

4. Reading the results: what the numbers mean

5. The three pressures worth measuring

A thorough diagnosis measures three pressures, not one:

• Main relief pressure — the system's ceiling. Tests the pump's ability to hold pressure and the relief valve's setting.
• Working / actuator pressure — measured at a specific function under load, it shows whether pressure reaches that function and helps localise a fault to a circuit.
• Pilot pressure — the control pressure that moves the spools. Low pilot pressure makes the machine feel slow and weak even with a healthy main pump, and it is a common and cheap-to-fix culprit.

Checking all three prevents the classic error of replacing a good main pump when the real fault is pilot pressure or a control valve.

6. When pressure is fine but the machine is slow: add a flow test

If pressure meets spec but the machine is slow, the fault is flow. A flow test (or a flow meter in the pressure line) measures how much oil the pump actually delivers at working pressure. A pump with worn pistons and shoe plate can still reach relief pressure at stall yet deliver well below its rated flow under load — which is exactly why the machine feels weak even though the gauge looks fine.

Flow testing is the step that turns "the pump is bad" from a guess into a measurement. Do it before you commit to a replacement; on a tandem-pump machine it also tells you which of the two pumps is down. Where the pump is confirmed faulty, a complete replacement such as the HPV145 pump for the Zaxis330 restores full flow without a rebuild turnaround.

7. Common mistakes that give false readings

• Testing cold. The number one error. Cold oil hides leakage and makes a worn pump look healthy.
• Testing with a blocked filter or a starved pump. A suction restriction looks exactly like pump wear — check the filters and suction line first.
• Judging without the specification. "It looks about right" is not a diagnosis; compare against the machine's data.
• Blaming the pump before the relief and pilot. A stuck relief or low pilot pressure mimics a worn pump and costs far less to fix.
• Replacing the pump without cleaning the system. Contaminated oil kills the new pump too. Change the oil and filters and flush as needed.

8. Buying and service information

• How to order: send the machine model and serial, the pump part number and a photo of the old unit; we confirm before dispatch.
• Testing: pumps are checked and measured before shipment; complete assemblies are bench-tested where applicable.
• Packing and shipping: heavy items ship in wooden export cases; air and sea freight quotable to your port.
• Warranty: 3–12 month warranty options depending on the part and grade.
• Contact: email service@gzotto.com, message us on WhatsApp at +86 153 0226 9990, or use our contact page. The full pump range is under excavator hydraulic pumps.

9. Frequently asked questions

What pressure should an excavator hydraulic pump produce?

It depends on the machine and the circuit. Always compare your reading with the machine's service specification rather than a general figure, and take it at normal operating temperature.

Can I pressure test a hydraulic pump myself?

With a calibrated gauge, the correct adapters, the machine's specification and a safe stall procedure, yes. Never open a pressurised system or hold a stall beyond the service procedure.

Why is my machine slow if the pressure is correct?

Because the fault is flow, not pressure. A worn pump can still reach relief pressure at stall but deliver too little flow under load — confirm with a flow test.

What does low pressure when hot mean?

It is the classic sign of internal leakage — usually worn pistons and shoe plate in the pump. Pressure that is low hot and cold alike more often points at the relief valve.

Do you supply pumps for tandem-pump machines?

Yes. We supply complete main pumps and rebuild parts for tandem and single-pump machines, with pre-shipment checking. Send the machine serial for confirmation.

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