Some important tips about brake fluids

Marcos Thadeu Lobo

Mechanical Engineer Graduated from the State University of Campinas (Unicamp). He currently works as an Associate Consultant at the company QU4TTUOR CONSULTORIA.

Brake fluids

Brake fluids – The fluid used in the brake system is a product of vital importance in mobile equipment. When we press the brake system pedal, the fluid acts as hydraulic fluid being compressed, activating the master cylinder and wheel cylinders, which, in turn, cause the brake shoes and pads to come into friction with each other. the brake drum or disc. The brake system fluid also works as a lubricating and anti-corrosive fluid for the metal parts of the brake system. The performance level of fluids for the brake system is specified by OEMs, by ABNT – Brazilian Association of Technical Standards and by DOT – Department of Transportation – USA, the most used specifications being DOT3, DOT4 and DOT5.1.

Figures 1/2 – Disc and drum brake systems

The main difference between DOT3, DOT4 and DOT5.1 brake system fluids is the Dry Boiling Point and Wet Boiling Point since, in brake systems, it is highly undesirable for the hydraulic fluid to boil as the gases are compressible and their presence can compromise the braking of mobile equipment and cause accidents. DOT5.1 brake system fluids have a Dry Boiling Point and Wet Boiling Point higher than DOT4 which, in turn, are higher than DOT3. However, each OEM will determine which product is most suitable for the mobile equipment’s brake system.

The DOT3 and DOT4 brake system fluids are hygroscopic, that is, they absorb water from the atmospheric air. The presence of moisture in brake systems can lead to boiling water (compressible gas), which is why some suggestions can be given in this regard:

Figure 3 – Typical constitution of a hydraulic brake system

1. Change the brake system fluid every 12 months or 10,000 km driven and this service must be performed by a qualified professional in order to avoid leaks.
2. Keep the brake system fluid reservoir well closed, regularly checking that there are no cracks in the compensation reservoir cap in order to avoid contamination by moisture.
3. Avoid storing brake system fluid in open containers as they may absorb moisture from atmospheric air and, when used, have the Dry Boiling Point and Wet Boiling Point below the specified.

Although the brake system fluid with DOT5.1 performance level has low hygroscopicity, it must also be changed at the intervals recommended by the OEM, as water pockets may form in the brake system circuit, which can cause corrosion of the brake system. metallic parts and failures in the activation of the brake system by boiling decanted water.

Figures 4/5 – Brake system fluid: regular replacement according to OEM

Brake system fluids are generally synthetic compounds based on polyglycols and glycol esters, being compatible with the elastomers used when the OEM recommendation is for a DOT3, DOT4 or DOT5.1 product. However, there are cases of mobile equipment where the brake system fluid recommended by the OEM is mineral-based oil. In these cases, the use of brake system fluids with performance levels DOT3, DOT4 or DOT5.1 will be highly harmful because the Kinematic Viscosity is not recommended ( ~ ISO VG 15 ) and the compatibility with the elastomers used is not adequate, leaks may occur. In these cases, a specific product recommended by the OEM must be used.

Figures 6/7 – Mineral-based oils: specific mobile equipment