This is a general, instructive guide to how the system works and what to look for. It is not a service manual, and it is not specific to your truck. Procedures, clearances, pressures, torque figures and service limits differ between makes, models and build dates, and the manual for the machine you are actually working on is the only authority on what is correct for it. Work to that manual, and to your employer's safety procedures.
A forklift hydraulic system does two unrelated jobs from one pump and one tank. It steers the truck, and it lifts and tilts the load. Those are separate circuits with separate relief valves and separate failure modes, and the single component that keeps them from interfering with each other — the flow control valve — is the one most people have never heard of.
What is in the system
Hydraulic tank, gear pump assembly, steering control unit, main control valve, lift cylinder, tilt cylinders, steering cylinder, a return filter and a breather. The tank carries a screen at its outlet, a drain plug at the bottom, and the breather on top that lets air in and out as the oil level changes with cylinder movement.
The steering circuit and the lift and tilt circuit are genuinely separate systems. They simply share the tank and the pump.
How the flow gets divided
The pump draws oil from the tank through the screen and delivers all of it to a flow control valve mounted on the pump's own end housing. That valve splits the flow into two streams, and it does so with a strict priority.
Steering is the primary flow and it is served first. The valve passes a fixed quantity of oil to the steering control unit no matter what the engine is doing. Whatever the pump produces beyond that becomes the secondary flow, and the secondary flow goes to the main control valve for lift, tilt and any auxiliary functions.
The mechanism is simple. Pump oil enters the valve chamber at the middle of a plunger and passes through an orifice in that plunger toward the steering port. Flow through the orifice creates a pressure difference across the plunger. When pump output exceeds what the steering circuit needs, that difference pushes the plunger against its spring and uncovers the port to the lift and tilt system. If pump output drops — because engine speed fell — the plunger moves back and reduces or shuts off the lift and tilt supply. Steering keeps its share throughout.
Priority flow explains a whole family of complaints. Lift speed falling off when the operator steers is normal behaviour, not a fault. Steering that stays consistent at idle while lift is slow is the system working as designed. And a truck that steers perfectly but will not lift has a healthy pump — the problem is downstream in the secondary circuit.
Inside the gear pump
Two gears in a close-tolerance body, one driven by a shaft splined to the engine, the other turned by the first. The pump mounts to the engine at the timing cover and turns whenever the engine runs.
The pumping action is often described wrongly. As the gear teeth separate at the inlet, they create a vacuum and oil enters. That oil is then carried around the outside of each gear, trapped between the teeth and the body wall, to the outlet — it does not pass between the meshing gears in the middle. At the outlet the teeth mesh again, displacing the oil out of the tooth spaces and into the outlet port.
The pressure-balanced bushings
The shafts either side of each gear run in bushings, and those bushings are not simply bearings. They carry drilled passages that route oil deliberately: inlet oil to lubricate the bushing and the gear face, and outlet oil to the back side of each bushing, away from the gears.
That outlet pressure behind the bushing pushes it against the gear face, maintaining the running clearance under load and keeping pumping efficiency up as pressure rises. The passages are arranged so pressure on each bushing stays balanced.
Two practical consequences. Bushings must go back exactly as they came out — same position, same orientation relative to the inlet. And the small seals in each bushing must be correct in number and position, because they define which passage sees which pressure. Get either wrong and the pump is quiet, turns freely, and does not build pressure.
The two relief valves
This is the second thing people get wrong, and it costs more time than anything else in the system.
The steering relief valve lives on the gear pump assembly, in the same end housing as the flow control valve. It limits pressure in the steering circuit only. When steering pressure reaches the setting, a poppet lifts off its seat against a spring. On later designs the relieved oil returns through an internal passage directly to the pump inlet; on earlier ones the housing has an external outlet returning it to tank.
The lift and tilt relief valve is inside the main control valve, a completely different component in a different place, with its own adjustment procedure. Trucks with a four-spool valve normally also carry a secondary relief valve.
So "the hydraulics have no pressure" is not a diagnosis. Steering weak with lift normal points at the pump-mounted relief. Lift weak with steering normal points at the main control valve. Both weak points at the pump itself, or at the oil.
Cavitation and aeration are different faults
Both make a gear pump noisy and the fault list treats them as separate causes, which is worth understanding because the fixes differ.
Cavitation is the pump being starved. It cannot draw oil fast enough, vapour cavities form in the inlet and collapse violently at the outlet. Causes are all on the suction side: low oil level, a restricted suction screen, or oil too thick for the temperature. Cavitation erodes metal and destroys pumps.
Aeration is air getting into oil that is otherwise flowing freely — a loose inlet fitting, a porous or collapsing suction hose, a return line discharging above the oil level and foaming the tank. The oil goes milky and the system feels spongy.
Both sound similar. The difference is that cavitation is a restriction problem and aeration is a leak problem, and a leak on the suction side draws air in without ever dripping oil out.
Wear and inspection
Worn or damaged seals are the most common reason a gear pump comes apart. Bushings, gears and shafts wear too, and all of them get measured during disassembly against the figures in the service data for that pump.
Nothing inside a gear pump is repairable. Worn or damaged parts get replaced, and if several main parts are worn the correct answer is a complete group rather than a collection of new parts running against old ones. Machining, lapping or dressing a gear face or a bushing destroys the clearances the pump depends on.
What gets measured: the bore of the pump body where the gears run — wear concentrates on the outlet side and is measured across the figure-eight bore — the outside diameter of the bushings, their length, and the bearing surface diameter on each gear shaft. Body bore wear is also assessed by how far around the circumference it extends.
Things that go wrong on reassembly
- Mark the housings before splitting them. An alignment mark across the valve end housing and the pump body takes five seconds and removes all doubt later.
- Note bushing orientation, and seal count and position, as they come out. There is no way to work this out afterwards from the parts themselves.
- The radius lines on the gear shaft ends must finish on the same end of the pump. Easy to reverse a gear and not notice.
- Tape the drive shaft splines before fitting the flange end housing. Splines cut a new shaft oil seal on the way through, and the pump then leaks from new.
- Remove the relief valve by its jam nut and the setting survives. Disturb the adjustment screw, acorn nut or jam nut and the whole relief pressure has to be reset with a gauge.
- Prime the pump before first start. Fit the hydraulic lines but leave the inlet fitting loose, fill the tank, and let oil run out around the inlet until it flows clean. Then tighten. A gear pump started dry damages its bushings in seconds.
- Oil every part on assembly, and keep everything clean — dirt is what killed the last set of seals.
Checking pressure and flow
Numbers are model-specific. Take relief pressures, pump flow and lift speeds from the specification data for the truck. The method:
Steering relief pressure
Connect a tachometer and fit a gauge with a gate valve at the test port on the steering circuit. Run the engine at a low set speed and work the hydraulics until the oil reaches operating temperature — a cold reading is meaningless because cold oil is thicker. Then turn the steering to full lock and hold it there, which forces the circuit over relief, and read the gauge.
Adjust at the relief valve screw: clockwise raises pressure, counterclockwise lowers it. Hold the screw while tightening the jam nut so the setting does not move, then fit the seal washer and acorn nut.
Pump flow
Fit a flow meter in the pump outlet line and follow the meter manufacturer's procedure, with the oil at operating temperature. Flow tells you what pressure cannot: a worn pump can still reach relief pressure at idle while delivering far too little volume to lift at a usable speed.
If relief pressure is correct and lift speeds are still slow, the pump is worn or the fault is elsewhere in the lift circuit. And a worn pump nearly always gets noisy before it gets slow — so listen to the truck before you connect anything to it.
Troubleshooting
| Symptom | Probable cause | Check and remedy |
|---|---|---|
| Pump noisier than normal | Oil level low or tank empty (cavitation) | Check the level and fill; look for the leak that caused it |
| Wrong type or grade of oil for the temperature (cavitation) | Drain the tank and refill with the specified oil | |
| Suction screen restricted (cavitation) | Clean or replace the screen | |
| Inlet fitting loose, or hoses drawing air (aeration) | Tighten the fitting, replace hoses, bleed the air out | |
| Pump bearings or gears damaged | Repair or replace the pump | |
| Capscrews holding the pump together loose | Tighten to the specified torque | |
| Pump loose at its mounting | Tighten to the specified torque | |
| Pump drive mechanism loose, worn or damaged | Check and replace as required | |
| Output below specification | Any of the causes listed under pump noise above | Work through those first — a worn pump nearly always gets noisy before it gets slow |
| Pump worn or damaged | If relief pressure is correct and lift speeds are still slow, the pump or another part of the lift system is at fault | |
| Pump leaks | Fittings or hoses on the outlet side loose or damaged | Tighten, or replace as necessary |
| Capscrews holding the pump together loose | Tighten to the specified torque | |
| Seals inside the pump damaged | Install new seals, or a new pump | |
| Steering pressure below specification | Relief valve out of adjustment | Adjust, or install a new relief valve |
| Relief valve damaged | Repair or replace the relief valve | |
| Pump worn | Repair or replace the pump | |
| Steering flow below specification | Pump worn, damaged or restricted | Check that the inlet hose is not soft, restricted or drawing air; replace the pump if needed |
| Relief valve out of adjustment | Adjust, or install a new relief valve |
Frequently asked questions
Why does lift slow down when I steer at the same time?
That is the system working correctly. A flow control valve on the pump gives steering priority, passing it a fixed quantity of oil before anything else. Only the surplus goes to lift and tilt. When steering takes its share the lift circuit gets what remains, so lift slows. It becomes a fault only if lift is slow when you are not steering.
Steering is fine but the mast lifts slowly. Is the pump worn?
Probably not. Steering has priority, so a genuinely worn pump usually shows up as weak steering as well. Steering normal with lift weak points at the secondary circuit — the main control valve and its own relief valve, which is a separate component from the relief valve on the pump. Check the lift relief pressure before touching the pump.
There are two relief valves? Which one do I adjust?
Yes. The steering relief valve is on the gear pump's end housing and limits steering pressure only. The lift and tilt relief valve is inside the main control valve and is adjusted separately. Adjusting the wrong one is the single most common time-waster on these systems — match the valve to the circuit that is actually weak.
The pump has got noisy. What should I check first?
Oil level, then the suction screen, then the oil grade for the temperature. Those three are the cavitation causes and they are all cheap. After that, check the inlet fitting and suction hose for air leaks, which cause aeration rather than cavitation but sound similar. Only then look at the pump itself — and remember a suction-side air leak draws air in without ever dripping oil out.
Can a worn gear pump still make full pressure?
Yes, and this catches people out. A pump can reach relief pressure at low engine speed while delivering far too little volume to lift at a usable speed, because pressure and flow are different measurements. If relief pressure checks out and the truck is still slow, put a flow meter in the outlet line rather than assuming the pump is good.
Can I lap or dress the gear faces to bring an old pump back?
No. A gear pump depends on very close tolerances between gears, body and bushings, and removing material destroys them. Worn parts get replaced, and when several main parts are worn the correct repair is a complete group rather than new parts running against old ones.
Do the bushings really have to go back the same way round?
Yes. The bushings carry drilled passages that route inlet oil for lubrication and outlet pressure to their back faces, which is what holds them against the gears under load. Orientation relative to the inlet determines which passage sees which pressure, and the number and position of the small seals in each bushing does the same. Reassembled wrong, the pump turns quietly and builds no pressure. Note it all during disassembly.
Before you order parts
Most gear pump work is a seal kit — shaft oil seal, snap ring, body and bushing seals, back-up rings and O-rings — since seals are the usual failure. Beyond that: bushings, the drive gear and shaft, the driven gear, and the flow control and relief valve internals with their O-rings and back-up rings.
System-side wear items are the return filter, the tank breather, the suction screen, and the hoses. The breather is worth a mention: a blocked one puts the tank under vacuum as cylinders extend and starves the pump, which presents as cavitation noise that no amount of oil will fix.