PARTS: Break fluid

By: Gary Worrall


To keep your fleet safe to drive - check your brake fluid

Bus fleet operators need to pay attention to not just the condition of the brake pads and rotors, but also the brake fluid to ensure vehicles remain safe to drive.

Although most vehicles sold in Australia use either DOT 3 or DOT 4 fluids, which comply with United States Department of Transport (DOT) regulations, Australia has more stringent standards, AS1960.1 G1 and G2, specifically in terms of the fluid’s boiling points.

While the US standard, which is used as an international standard, allows clean fluid to boil at 205C (DOT 3) or 230C (DOT4), the Australian standard raised this to 230C for G1, the DOT3 equivalent, and 260C for G2, the DOT4 equivalent.

For fluid with a 3.7 percent water contamination, these figures drop to 140C for DOT3 and G1, and 155C for DOT4 or G2 fluids.

This relates to real world driving, as brake fluids absorb water from the atmosphere, through the porosity of brake hoses or from the venting of the fluid reservoir.

Importantly, water content reduces the boiling point of the fluid, which impacts on the ability of the brake system to stop a vehicle, especially when carrying heavy loads or if the ABS system is actuated.

This happens because modern brake systems rely on brake fluid to be incompressible, that is, as the pedal is pushed, the fluid passes the pressure down the lines to the brake pistons, which then clamp the pads onto the rotors, providing the stopping force.

The fluid is compressed when it is heated, making boiling points important for safe braking.

The higher the water content in the fluid, the lower the boiling point, which in turn reduces the amount of pressure passing from the driver to the brakes so that less clamping force is applied to the discs, lengthening stopping distances.

With ABS systems using multiple brake applications and releases per second to apply maximum force while allowing the driver to maintain steering control, more heat is generated so that if the fluid boils and becomes vapour, the brakes do not receive the message to brake as heavily. It leads to the same result of longer stopping distances.

Bendix says this is why vehicles can suffer significant brake failure even if the brake components are in perfect condition and when tested later after the fluid has returned to its liquid state and the brakes function perfectly.

Although specific fluid change intervals vary between manufacturers, fluid can be tested for water content in a workshop, to ensure maximum braking performance under all conditions.

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