Measure drift with the oil at operating temperature, the bucket loaded, the engine off and the control lever in neutral, and record how far the boom falls in a fixed time. Express the result in millimeters per minute at the cylinder rod, not in fingers of travel, so it can be compared with the specification and with the next test. A machine that only drifts when cold is not drifting by this definition. If the measured rate exceeds specification, isolate the cylinder before condemning the valve: cap both cylinder ports and repeat. A cylinder that still drifts with its ports capped is bypassing internally; one that stops is passing oil through the control valve.
What you will notice
| Symptom | What it usually points to |
|---|---|
| Boom sags overnight but holds a load all shift | Slow drift, usually below the rate that matters. Measure it before repairing anything. |
| Drift speeds up as the oil gets hot | Internal leakage that grows as viscosity falls. Cylinder seals or spool clearance, and the test will say which. |
| Drift only with a load in the bucket | Load-dependent leakage, typically the piston seal or a relief valve that is not seating. |
| Boom and arm both creep | Shared valve leakage, or two worn components. Isolate each cylinder separately rather than treating it as one fault. |
| Drift that stops when the engine is running | Not drift in the measurement sense, because pump flow is compensating. The static test below will read close to zero. |
Most likely causes
| Cause | How to confirm it |
|---|---|
| Piston seal worn or scored | Internal bypass past the piston. Confirm by capping the cylinder ports and repeating the timed test. |
| Rod seal leaking externally | Visible oil on the rod. External leakage does not make a boom drift, but it is a reason to open the cylinder anyway. |
| Control valve spool and bore clearance | Oil passes to tank through the center. Drift that stops with the ports capped points here. |
| Circuit relief valve not seating | A relief that weeps is a permanent leak path. Check it with the function stalled and a gauge fitted. |
| Holding valve leaking where one is fitted | A load lock that does not seat lets the boom down at a steady rate regardless of temperature. |
Reading the test result
| Result | What it means |
|---|---|
| Under specification, hot and loaded | No drift fault. Record the number and repeat it at the next service. |
| Over specification, stops when ports are capped | Control valve, relief valve or holding valve. The cylinder is not the problem. |
| Over specification, continues with ports capped | Cylinder internal bypass. Remove and reseal. |
| Much worse hot than cold | Clearance-related leakage. Expect both cylinder and valve to be marginal. |
| Drift in one direction only | Asymmetric leakage: spool land wear on one side, or a single seal failure. |
How to confirm it
- Bring the oil to operating temperature
Work the machine for at least half an hour, or until hydraulic oil reaches the temperature it runs at in service. Cold oil hides internal leakage, and a cold reading will pass a cylinder that fails by the afternoon.
- Set the machine on level ground and mark a reference
Boom at a fixed height, arm and bucket in a known position, and a mark on the cylinder rod. Level ground matters more than most people expect: three degrees of slope changes the result by more than the tolerance allows.
- Load the bucket, stop the engine and leave the cab
Use a load close to what the machine carries in normal work. Engine off, lever in neutral, nobody in the operator seat. A static test on a live hydraulic system is how people get hurt.
- Time the fall and measure at the rod
Record the travel over two minutes with a stopwatch and a rule against the rod, then convert to millimeters per minute. Two minutes separates real drift from seal settling; ten seconds does not.
- Repeat hot, then again cold
The difference between the two readings is the diagnostic value. A rate that doubles when hot is a clearance problem; a rate that is the same hot and cold is more likely a valve seat.
- Cap the cylinder and repeat to isolate it
Blank both ports at the cylinder, refit the load and time the same movement. This is the only test that separates a cylinder from a valve without dismantling either. Guangzhou Shenyue Machinery Technology Co., Ltd. supplies seal kits, piston and rod seals, relief valves and holding valves for common excavator cylinder series, identified from the cylinder code and rod diameter. Stock is held as 3,000+ part types on the shelf, with 100,000+ part numbers available to order.
What to do about it
If the cylinder bypasses with its ports capped, reseal it properly Replace the piston seal, wear rings and rod seal as a set, and measure the bore and the rod before fitting anything. A scored piston reused in a fresh kit will leak again within a few hundred hours.
If drift stops with the ports capped, work on the valve Check spool clearance, the circuit relief seat and any holding valve. The cheapest correct order is relief seat first, holding valve second, spool third, because that is also the order of cost.
If the rod seal is leaking but the drift rate is within specification, say so An external leak is a different fault with a different repair. Quoting a full cylinder reseal for a weep that does not affect holding is how a customer loses confidence in the diagnosis.
Record the before and after numbers and give them to the operator The same test, hot and loaded, is the only acceptance criterion that means anything. A repair signed off on feel will be argued about within a month.
Parts you may need, by grade
| Part | Grades commonly quoted |
|---|---|
| Cylinder seal kit | Genuine; OEM-quality; aftermarket. Kit contents listed on the quotation. |
| Piston and wear ring | Genuine; OEM-quality. Bore measurement required before ordering. |
| Rod seal and wiper | Genuine; OEM-quality; aftermarket. Material stated for high-temperature service. |
| Holding valve and circuit relief | Genuine; OEM-quality. Setting verified on a bench where adjustable. |
Need the parts this covers?
Send the machine model, serial number and the part numbers you need. We confirm fitment before payment and state the grade of every line — genuine, OEM-quality or aftermarket.
Request a quoteFrequently asked
How much boom drift is normal?
It depends on the model, and the specification is written in millimeters per minute at a stated oil temperature, which is why the test above is run hot. A number quoted from feel, or from how far the boom moved overnight, cannot be compared with anything. If you do not have the figure, record your own baseline while the machine is known good and compare against that.
Why measure at the rod instead of at the bucket?
Because the rod is where the leakage is happening and where the measurement repeats. Bucket height also moves with linkage clearance, so a bucket measurement mixes two faults and neither can be quantified. Convert rod travel to volume only if you are comparing against a leakage specification written as flow.
Does a leaking rod seal cause the boom to drift down?
No. The rod seal keeps oil inside the machine; it does not hold the boom up. A boom that drifts with a dry rod is bypassing internally, past the piston seal or through the valve. Replacing a rod seal because the boom drifts is one of the more common wasted repairs in this trade.