Hydraulics — the Hidden Cost That Explodes After Purchase
The hydraulic system in a press, press brake, or injection molding machine is one of the most expensive components to repair. Replacing a hydraulic pump: 3,000-15,000 EUR. Rebuilding a cylinder: 800-4,000 EUR. New valve block: 2,000-8,000 EUR. And it all starts with a small leak the seller wiped away before your arrival.
As a procurement agent, Hutnia sees this regularly: the machine sits on a clean floor, the seller says "no leaks," and after 30 minutes of operation under pressure, stains appear on the concrete. Below, we describe how to search for leaks systematically — before you sign the contract.
Visual Inspection — Before You Start the Machine
The first test happens with the machine powered off. You need no tools — just a flashlight and clean hands.
Where to look:
1. Hydraulic cylinders — run your finger along the piston rod. A thin, uniform oil film is normal. Drops or wet streaks — seal needs replacement.
2. Hose connections — every threaded fitting, every quick-connect coupling. Look for wet rings around the nuts.
3. Hydraulic pump — check the underside of the pump and the connection to the motor. Oil under the pump means the shaft seal must go.
4. Valve block — complex because of the many ports. Run your finger across each valve. Wet means leaking O-rings.
5. Hydraulic tank — check the oil level at the sight glass. If it is below minimum on a cold machine — oil is escaping somewhere.
6. Floor under the machine — the easiest test. Ask the seller to move the machine or remove covers. Oil stains on concrete tell you everything.
Red flag: Freshly mopped floor under the machine plus solvent smell means the seller cleaned up leak traces before the viewing.
Acoustic Inspection — Listening Under Pressure
Internal leaks leave no stains on the floor but betray themselves through sound. Start the machine and let the hydraulic system reach operating pressure.
What to listen for:
1. Hissing at valves — a quiet, constant hiss means internal flow past the valve seat. The machine loses pressure and the pump works harder.
2. Pump cavitation — metallic cracking or rattling. The pump is drawing air through a leaking suction line. Untreated cavitation destroys the pump within weeks.
3. Cylinder knocking — during direction changes, the cylinder should not knock. Knocking means air in the system or worn cylinder guides.
4. Valve block buzzing — proportional valves produce a faint hum. Loud buzzing means contaminated oil or worn solenoid coils.
Mechanical stethoscope method: Buy a workshop stethoscope for 10-15 EUR (metal rod with a listening head). Place it on the pump housing, valve block, and cylinders. Leak sounds become many times clearer than what the unaided ear picks up.
Pressure Test — 10 Minutes That Can Save Thousands
If the machine has a pressure gauge on the hydraulic system (most do), run this simple test:
- Start the machine and let it build operating pressure.
- Note the gauge reading.
- Shut off the hydraulic pump (not the entire machine — just the pump).
- Watch the gauge for 10 minutes.
Interpretation:
- Drop up to 5% in 10 minutes — normal loss through check valves
- Drop 5-15% — internal leaks, but machine can operate. Negotiate the price.
- Drop above 15% — serious internal leaks. Complete diagnosis required.
- Drop to zero in under 2 minutes — critical defect, likely a failed check valve or burst internal line.
The Oil Tells the Truth — Visual Fluid Analysis
Ask the seller for an oil sample. Pour it onto a white sheet of paper and evaluate:
- Amber/golden — fresh or well-maintained oil
- Dark brown — spent oil, but normal at change intervals of 2,000-4,000 hours
- Black with metal particles — internal wear on pumps or cylinders. Requires filtration and further diagnosis.
- Milky/cloudy — water in the oil. Likely a damaged oil cooler or condensation from large temperature swings. Serious problem — water destroys pumps and valves.
- Burnt smell — overheated oil. The pump ran at excessive pressure or with insufficient cooling.
A professional oil analysis at a lab costs 80-150 EUR and provides the full picture: metal wear, water content, viscosity, contaminants. Hutnia commissions these analyses for machines above 50,000 EUR.
Repair Costs — What a "Small Leak" Really Costs
Sellers downplay leaks. "It is nothing, just an O-ring for 2 EUR." In reality:
| Repair | Cost (EUR) | Downtime |
|---|---|---|
| Cylinder seal replacement | 200-800 | 4-8 h |
| Hydraulic hose replacement | 50-300 | 1-2 h |
| Pump rebuild | 1,500-5,000 | 2-5 days |
| New pump | 3,000-15,000 | 1-3 days |
| Valve block rebuild | 2,000-8,000 | 3-7 days |
| Cylinder replacement | 1,000-6,000 | 1-3 days |
Total hydraulic repairs on a neglected machine can reach 20,000-30,000 EUR. A leak is not cosmetic — it is a warning sign.
From Observation to Decision
The hydraulic check is the third pillar of machine evaluation — alongside the spindle check and the geometry check. If the machine passes all three tests, a complete test run with a mechanic is the logical next step.
Remember: hydraulic leaks only get worse. A minor seal weep today is a breakdown in three months. Better to know before the purchase than after the first month of production.
Book an initial consultation Step 0 for 49 EUR — fully deductible from the 500 EUR mandate. Schedule now
