A mast that sinks under load or oil on a cylinder are common findings, but neither automatically means worn seals. This guide shows how to tell the cylinder, the control valve and the lines apart, which seals work inside a cylinder and which damage patterns point to which cause. We give no drift limits, because they belong to the manual of the specific truck.
Narrow it down before you disassemble
A service manual from a mast manufacturer (Lift Technologies) lists four possible causes for cylinders that drift. These are an external leak in the pressure line, a defective truck control valve, an external leak at the cylinder head, and a leaking piston check valve. Only the third cause is fixed there by replacing all cylinder seals. To check the control valve, the manual names a stroke to full lift height.
The manufacturer sets the acceptable drift and the test conditions for each model. Figures from other trucks do not transfer, so take both from the manual for your truck.
Safety before any inspection
Hydraulic oil escaping from a crack or gap is under high pressure. DGUV Information 209-070 states that such jets can cause very severe injuries and that even protective gloves do not give full protection. It advises searching for leaks at reduced system pressure where possible, with protective equipment and with aids such as blotting paper, and always keeping a distance from the leak. If oil penetrates the skin, see a doctor immediately.
Before work on the system, the DGUV lists five steps. Cut the energy supply, secure against restart, depressurize the system and lower or support raised loads, verify that the pressure is gone, and prevent hazards from neighboring equipment. Before a cylinder is replaced, it must be free of forces, and pressure is relieved on both the piston side and the rod side. The mast manufacturer's manual adds a warning against working on the mast with a raised load that is not supported.
Under OSHA 29 CFR 1910.178 paragraph (q)(1), a truck that is not in safe operating condition is removed from service, and repairs are made by authorized personnel. Under paragraph (q)(5), replacement parts must be equivalent in safety to the original design. It sets no leakage limit.
Which seals work in the cylinder
| Component | Job | Conspicuous when |
|---|---|---|
| Rod seal | Keeps oil inside and seals against the rod or plunger | Oil film or drops at the cylinder head, wet rod |
| Wiper | Keeps dirt, chips and moisture off the rod | Scores on the rod, extruded or cracked lip |
| Guide ring or bearing | Guides piston and rod, takes side loads, prevents metal contact | One-sided wear, galling |
| Piston seal | Separates the pressure chambers in the cylinder | Drift without visible oil |
| Back-up ring | Stops the seal from being pushed into the gap | Frayed, nibbled seal edge |
Trelleborg describes the wiper as protection for seals and wear rings against contamination, and the guide ring as the part that absorbs transverse forces and prevents metal-to-metal contact. Parker's cylinder bulletin adds that a faulty wiper lets dirt in, which shows up as score marks on seal and bearing.
Reading the findings
| Finding | Likely cause | Check or action |
|---|---|---|
| Oil at the cylinder head, wet rod | Rod seal or wiper worn, rod damaged, side load | Clean the rod, watch it over several strokes, inspect the rod surface |
| Mast drifts down, outside dry | Piston seal, piston check valve or control valve | Separate cylinder from valve as the manual describes |
| Mast drifts down, oil at a fitting or line | Fitting or line leaking | Tighten or replace the line while depressurized |
| Scores or scratches on the rod | Dirt from a faulty wiper or contaminated oil | Judge the rod, check oil condition and wiper |
| Seal and guide worn on one side | Side loading or misalignment | Find and remove the source of the side force |
| Seal hard, cracked, brittle | Temperature too high or too low | Check operating temperature and seal material |
| Seal swollen, shrunken or sticky | Incompatibility with the oil | Check that the oil is approved for the seal material |
| Seal edge nibbled | Gap too large or pressure peaks | Check gap, back-up ring and pressure peaks |
| Spiral cuts on a round seal | Seal rolls and slides at once, for example under side load or on a rough surface | Check surface finish and concentricity of the sliding parts |
| Jerky stroke | Air in the cylinder, relief valve, bent plunger | Bleed, check valve and plunger |
The rows on temperature, oil, gap and spiral cuts rest on the Parker cylinder bulletin about rod seal leakage and the Parker O-Ring Handbook. Parker also notes that some low-friction seals need about 100 strokes after installation before the rod runs dry. A thin oil film right after a repair is therefore not necessarily a fault.
When a seal kit is enough
An overhaul kit is enough when the seals are the cause and the sliding surfaces are undamaged. The mast manufacturer's manual has the rod, plunger and piston inspected for nicks and burrs. Small nicks that pass no oil under pressure are dressed there with fine emery cloth of 400 grit. If they cannot be removed that way, the part is replaced. The same procedure replaces the retainer and piston seals together with back-up rings, round seals and bearing, not single seals.
A seal kit is not enough for a bent or deeply scored rod, a worn guide or a damaged cylinder tube. Parker names damaged rods and rod bearings as the most common cause of rod seal leakage. A new seal is quickly destroyed on a damaged surface, so remove the cause first, such as side load, dirt or unsuitable oil.
Before ordering, compare the kit contents with the manufacturer's parts list.
Notes on replacement
The DGUV requires seals of leaking cylinders to be replaced only as the manufacturer specifies, and cleanliness at all times when working on the cylinder. The mast manufacturer's manual caps open fittings and plugs the hoses.
Three points matter during assembly. The seal direction must be right, because according to Lift Technologies a kit installed backwards stops the cylinder from working properly. Edges over which a seal slides must be deburred and have a lead-in chamfer. Threads in the path of the seal are taped, following Parker. Torque values come only from manufacturer data. Even within one series, Lift Technologies gives retainer torques that differ by a multiple.
Bleeding follows. Air in the cylinder is compressed on the first extension and can rupture the cylinder, warns the mast manufacturer's manual. It calls for several strokes without load and then strokes with half load until the cylinder extends smoothly. The DGUV adds that transport plugs must be removed before installation and that bleed screws are closed only once the oil runs out free of bubbles.
Matching parts from the catalog
The kits below are listed in the catalog under these names. Fit does not follow from the number alone. Compare the nameplate and part number before ordering.
- For Toyota lift cylinders, the catalog lists the lift cylinder overhaul kit 04651-10060-71.
- For Toyota tilt cylinders, it lists the tilt cylinder overhaul kits 04651-10132-71 and 04655-03010-71.
- For rod leakage, Crown has the rod seal service kit 126903 listed. A single Crown piston seal is 064216-005.
- Clark lists the cylinder packing kit 1811309.
- Hyster lists the overhaul packing kit 2307177 and the tilt cylinder overhaul kit 1329757.
- TCM lists the overhaul packing kit 27248-49801, and Yale the overhaul packing kit 5051630-01.
FAQ
Is a seal kit enough if the mast drifts down slowly?
Only if the cylinder is the cause. Fittings, the control valve or the piston check valve can also let the mast sink. Narrow down the cause with the truck's manual before you take the cylinder apart.
Can I keep working with a leaking cylinder?
OSHA requires a truck to be removed from service when it is not in safe operating condition. Whether a particular leak counts is decided by the manufacturer's instructions and a competent assessment. Never work under a raised load that is not supported.
Why does the cylinder leak again soon after the seal change?
Often the cause was left in place, for example a scored rod, side load or contaminated oil. Installation damage at sharp edges or a seal fitted backwards also occurs. A thin oil film during the first strokes can be normal.
Do you need one of the parts mentioned? We deliver the matching part with a quote the same day, and express shipping is possible.