The hydraulic system of a truck makes it possible to perform heavy-duty tasks efficiently and in a controlled manner. Examples include applications such as tipper trucks, truck-mounted cranes, walking floor trailers, and container systems. In this blog, we explain how the hydraulic system works, which components it consists of, and why a properly functioning system is essential for the daily operation of a truck.
A hydraulic system converts mechanical energy into fluid pressure (hydraulic pressure) and uses that pressure to generate force. Because oil is hardly compressible, large forces can be generated with relatively compact components. This makes hydraulics ideal for applications where lifting, tilting, pressing, or driving is required.
In practice, hydraulics on trucks are mainly used for superstructures and auxiliary systems, such as a truck with a crane, tipper, or hook lift. If you work with a body or PTO drive, the hydraulic system is often the “beating heart” of the entire setup.
Although systems vary by body type and brand, you will almost always encounter the same building blocks. The core consists of:
Hydraulic pump: builds pressure and moves oil (flow).
Drive system: often via the PTO (Power Take-Off) on the gearbox, sometimes electric/hydraulic.
Hydraulic tank: storage, ventilation, cooling, and contamination control.
Filters: protect the pump and valves from wear caused by dirt or metal particles.
Valves/control blocks: regulate direction, pressure, and flow (e.g. lifting/lowering).
Hydraulic hoses and lines: transport oil under high pressure.
Cylinders: convert pressure into movement (linear or rotational).
Couplings/quick couplers: for connecting bodies, trailers, or auxiliary equipment.
You can think of the system as a closed loop:
The pump draws oil from the tank and pushes it into the system. Through a control valve, the oil is directed to the correct side of a cylinder (for example, the lifting cylinder of a tipper). By building pressure, the cylinder extends and delivers force. The returning oil flows back to the tank through return lines.
An important detail: the system operates not only on pressure, but also on flow (liters per minute). Pressure mainly determines “how much force,” while flow determines “how fast.” This means a truck can be hydraulically strong but still lift slowly (flow too low), or move quickly but struggle with heavy loads (pressure loss or incorrect settings).
This is the most common source of confusion in troubleshooting.
Pressure (bar): the “force” in the system. Low pressure may indicate pump wear, leakage, incorrect pressure relief valve settings, or internal leakage in cylinders/valves.
Flow (l/min): the “speed” of movement. Low flow may indicate insufficient pump output, low RPM, clogged filters, small line diameter, or valves not fully opening.
In practice: if a tipper suddenly moves slower but still has power, you often look at flow (filter, pump, blockage). If it wants to move but stalls under load, you are more likely dealing with pressure loss (leakage, valve, pump).
Hydraulic oil is not just “any oil.” The right choice depends on temperature, load, pump/valve type, and body specifications. The most important points are:
Use the correct viscosity (e.g. ISO VG class) for your conditions.
Do not mix oils with different additive packages or chemistry unless explicitly allowed.
Work clean: contamination in the oil is one of the main causes of wear and sticking valves.
Oil level and ventilation also matter. Too little oil can cause cavitation (air bubbles collapsing in the pump), leading to noise, damage, and pressure loss. Too much oil or poor ventilation can cause foaming—and foam behaves like compressible fluid, which is exactly what you want to avoid in hydraulics.
Most issues fall into a few recognizable categories:
1) External hydraulic leakage
You can see oil on hoses, couplings, cylinder rods, or valve blocks. This is often relatively easy to detect, but can have major consequences (pressure loss, contamination, fire hazard). Pay extra attention to stress points: bends, areas where hoses rub, and couplings that are frequently disconnected.
2) Internal leakage
A tipper slowly lowers or a cylinder cannot hold its position. This may be caused by wear in the cylinder (seals) or a valve that leaks internally.
3) Slow operation
Often related to low flow due to pump wear, clogged filters, oil that is too thick in cold conditions, or a valve that does not fully engage.
4) Overheating
Hydraulics generate heat due to losses (friction, restrictions, valve slip). If oil becomes structurally too hot, viscosity drops, accelerating wear and leakage. Causes include continuous pressure load, incorrect settings, undersized lines, or a cooler that is not functioning properly.
5) Noise: whining, rattling, or squealing
This often indicates cavitation, air in the system, low oil level, or a suction-side issue (blockage or leak).
A pump usually wears gradually. Common signs include:
The system responds slower than normal (reduced flow).
The system can no longer reach the required pressure under load.
Increased noise (cavitation/bearings) or faster oil heating.
Metal particles visible in the filter (bad sign: immediate inspection required).
Do you have questions after reading this blog? We are happy to help you. Contact us by phone or start a chat.
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