You hit the hydraulic stop. The pump gauge should collapse to zero instantly and the counterbalance gauge should sit at system pressure, locked behind the check. Instead the pump gauge holds for a couple of seconds, you hear fluid rushing back through the manifold, both gauges drop to zero, and the counterbalance gauge then climbs back up to system pressure on its own.
That sequence is one fault, not three. The pilot operated check valve is still open when the pump stops. Start there.
Read the sequence before you touch a wrench
The counterbalance cylinders are mounted rod end down and carry the head weight so the axis motor does not have to. That load is always pushing oil out of the cylinders. The only thing standing between that load and the tank is the PO check.
When the pump shuts down and the check is still piloted open, the load drives oil backwards through the supply line and spins the pump as a motor. That is the rushing noise and that is why the pump gauge holds for a second or two instead of dropping. The gauges only go to zero once enough oil has moved. When the pilot signal finally bleeds away, the check snaps shut, the cylinder load re-pressurises the trapped volume, and the counterbalance gauge jumps back to system pressure. Faster recovery after you fitted the new valves means the new poppet seats better once it finally gets the command to close — it does not mean the closing command is arriving on time.
| What you see | What it means |
|---|---|
| Rushing noise at shutdown | High-velocity flow across the open PO check poppet |
| Counterbalance gauge drops to zero then returns | Check closes late; cylinder load repressurises trapped oil |
| Counterbalance gauge never drops (correct behaviour) | Check closed on command; load held at the poppet |
| Rush starts as pressure passes a fixed value every time | Accumulator dumping at nitrogen precharge, not the check |
Check 1: the Y drain line
Look at the check valve mounting surface. The two smallest ports in the block are X and Y. X is the pilot pressure that opens the poppet. Y is the drain that lets the pilot piston chamber empty so the poppet can reseat. A PO check closes no faster than Y can dump.
On this machine Y runs back to DR2 on the power unit as a separate line. Confirm that on your own manifold before you trust it — some layouts route the drain into shared return plumbing that also feeds the Y-axis cable conveyor, and any return-side pressure there shows up directly as backpressure on the pilot chamber.
- Trace the
Yport line physically from the block to its termination. Do not read it off the schematic alone. - Confirm it lands at
DR2on the power unit and not into a pressurised or shared return header. - Crack the line at the manifold end and check that it drains freely. A slug of trapped oil or a full line will not let the pilot chamber empty.
- Check the line ID and the fitting bores. An undersized drain or a restrictor fitting left in from a different build starves the vent path.
- Look for a plugged or partially seated fitting at the block face. Debris in that small bore is common on a new install.
Free drain, fault still there: go to check 2.
Restricted, blocked, or wrongly routed drain: that is the fault. Fix it and go to verification.
Check 2: where the pilot signal comes from
If Y can vent, the poppet is being held open because X is still pressurised. Put a gauge on the pilot line and watch it against the pump gauge during shutdown.
- Pilot decays with the pump but the check stays open: the pilot piston is sticking in the bore. You have already replaced the valve once, so suspect contamination carried in from commissioning rather than the valve itself.
- Pilot holds pressure after the pump stops: something is keeping the pilot line alive. On a circuit with an accumulator, the stored volume is the obvious candidate. Find what the pilot line tees off and whether that branch sees accumulator pressure after shutdown.
- Pilot never fully reaches zero: you have a trapped-volume problem, not a valve problem. Look for a check or a closed directional valve isolating the pilot branch from tank.
A control valve that fails to centre or de-energise on shutdown keeps the pilot alive for exactly the kind of interval you are seeing. Verify the shutdown command actually drops the solenoid, not just the pump contactor.
Check 3: separate the accumulator from the check valve
The nitrogen accumulator will produce its own rushing noise, and it does it at a repeatable point. As stored oil discharges, pressure falls until it reaches the nitrogen precharge, and the remaining gas expansion pushes the last of the oil out in a burst. That is a free, noninvasive way to read approximate precharge: watch the gauge and note the pressure at which the noise starts or the needle takes a step.
If the noise always begins at the same gauge reading, you are hearing the accumulator dump, not the check valve. That is normal. It is not your fault. Your fault is the two-second pressure hold and the counterbalance gauge dropping to zero.
Check 4: gauge range and schematic agreement
Before you condemn any component, make sure the instrument can read the circuit. If the schematic calls the counterbalance pressure 5–5.5 MPa (50–55 bar) and the installed gauge tops out at 16 bar, that gauge is pegged and everything you read from it during troubleshooting is noise. Fit a gauge with a full-scale rating above the schematic value — conventional practice is normal operating pressure around the middle of the scale.
Reconcile every gauge and every pressure setting against the schematic before continuing. Also confirm the pressure reducing valve setting matches the drawing. If the reduced pressure is set above what the circuit expects, the pilot ratio changes and the check will hold open longer than intended even with a clean drain.
Fix and verify
For the drain-restriction branch, which is where this fault normally lands:
- Isolate the hydraulics, lock out the machine, and mechanically block the head so the counterbalance cylinders are not the only thing holding the load.
- Bleed the accumulator to zero before opening any line. It stores energy after the pump is dead.
- Disconnect the
Ydrain at both ends. Flush it, check the bore of every fitting, and remove any restrictor that is not on the drawing. - Reroute the drain to
DR2as a dedicated line if you find it teed into a shared or conveyor-side return. - Refit with line ID at least equal to the manifold
Yport bore. - Restore power, cycle the axis several times to purge air, and reset the accumulator precharge to the drawing value if you disturbed it.
Verification, with a correctly ranged gauge on the counterbalance side: shut the hydraulics off and watch both gauges. The pump gauge must fall to zero immediately with no hold interval. The counterbalance gauge must not move — no dip, no recovery. There must be no rushing noise attributable to reverse flow through the manifold, and the pump shaft must not turn backwards. Repeat the shutdown at least five times, including from a fully raised head position, since a slow pilot bleed will sometimes pass a single test and fail when the load is highest.
Two things that waste time here. Replacing the PO check again — you have already proved a new valve does not fix a drain problem, and the only change you got was a cleaner seat once it finally closed. And chasing the rushing noise as if it were the primary fault; it is a symptom of flow you have already located by watching the gauges.
FAQ
Why does hydraulic pressure hold for a few seconds after I shut the pump off?
The pilot operated check valve is still open, so the load on the counterbalance cylinders drives oil backwards and motors the pump. Pressure holds until the pilot signal bleeds off and the poppet seats.
Why does my counterbalance gauge drop to zero and then climb back to system pressure?
The check closes late. Oil escapes past the open poppet until it seats, then the cylinder load repressurises the trapped volume and drives the gauge back up.
Why does a pilot operated check valve close slowly even when it is brand new?
Because closing speed depends on the Y drain venting the pilot chamber, not on the valve internals. A blocked, undersized, or wrongly routed drain keeps the pilot piston loaded and a new valve behaves identically to the old one.
Why do I hear fluid rushing at a consistent pressure during shutdown?
That is the nitrogen accumulator discharging its last oil as system pressure reaches the precharge value. Note the gauge reading at that point as a rough precharge check; it is normal and separate from the check valve fault.
If the Y drain is confirmed clear and correctly routed to DR2, the pilot line decays with the pump, and the fault still repeats across multiple shutdowns, stop swapping cartridges. Take your gauge traces and the machine schematic to the machine builder and the valve manufacturer's technical support, since at that point you are looking at either a circuit design deviation from the drawing or a manifold porting defect on this specific build. Note that this is the first of five identical installs to behave this way — that comparison is the strongest evidence you can hand them.