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November 10, 2025

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Foaming in Wet Brake Systems: Causes, Consequences, and Prevention

Wet brake systems are commonly used in heavy equipment such as agricultural tractors, wheel loaders, excavators, and forklifts. Unlike dry brakes, they operate fully submerged in oil. This oil cools, lubricates, and protects the friction plates during braking.A stable oil is crucial for reliable brake performance. When foaming occurs, the oil loses its effectiveness, leading to issues like pressure loss, wear, and even system failure. This article explains the causes of foaming, its consequences, and how to prevent these risks.
How Does Foam Form in Oil?

During operation, oil is pushed through valves, filters, and narrow channels at high speed. This causes turbulent flow, entraining air. If this air cannot escape quickly, foam is formed: a mixture of air bubbles and oil.

Foaming primarily occurs due to low oil levels, topping up with unsuitable oil, or using aged oil with depleted additives. Even switching to a different viscosity can alter the flow and trap air. Well-designed systems are also susceptible if maintenance and oil usage are not optimal.

 

Consequences
  • Reduced Pressure Transfer
    Air in the oil is compressible, slowing down the build-up of brake pressure. This results in a spongy brake feel, longer stopping distances, and unpredictable cylinder behavior.
  • Cavitation in Pumps and Valves
    Small air bubbles implode under pressure, causing microscopic impacts on metal surfaces. This leads to wear, leaks, and reduced pumping capacity.
  • Poorer Lubrication and Heat Dissipation
    Foam displaces oil at contact points, forming an insulating layer, increasing friction and temperature. As a result, oil deteriorates faster, increasing the risk of damage to gears and bearings.
  • Accelerated Oxidation
    Exposure to oxygen accelerates oil oxidation. Sludge forms, oil becomes acidic, and additives lose their effectiveness.
Preventing Foam Formation
  • Use of Antifoaming Agents
    Antifoaming agents quickly eliminate air bubbles, keeping the oil stable and preserving pressure, lubrication, and heat dissipation. This is crucial for combined systems like wet brakes with hydraulics and transmission.
  • Choose the Right Oil
    Use only oil suitable for wet brake systems and meeting OEM specifications such as TO-4, JDM J20C, CNH MAT 3525, or MF CMS M1145. UTTO and STOU oils with proven antifoaming capacity and the right friction modifiers are most suitable.
  • Avoid Mixing Oil Types
    Combining different oils can cause additive conflicts, leading to loss of antifoaming agent effectiveness. Therefore, use only the prescribed oil.
  • Perform Maintenance Timely
    Change oil according to the schedule. Regular top-ups are not a substitute for changing oil as additives like antifoaming agents deplete over time.
  • Check Oil Condition
    Watch for signs like foam in the sight glass, milky oil appearance, or a fluctuating brake pedal. If in doubt, a Eurol Lube Check can clarify issues related to air entrapment, oxidation, or water contamination.

 

Foam formation poses a serious threat to the reliability of wet brake systems. By using the right oil, performing maintenance on time, and paying attention to antifoaming agents, you can prevent malfunctions and wear.

If you are unsure about the correct oil for your system, contact our inside sales department or your account manager. Want to know if your oil still meets requirements? Request a Eurol Lube Check. Prevention is always cheaper than downtime.