Brake fluid change
The majority of modern brake systems use synthetic brake fluids to transmit the hydraulic pressure generated in the brake master cylinder to the calipers and brake cylinders. The properties of brake fluids are therefore of vital importance to the overall brake system.
Brake fluids should not be referred to as “brake oil”, as the slightest contamination with oil will destroy the rubber parts in the brake system and could cause complete failure of the brake system.
The chemical and physical properties of brake fluids are defined in regulation SAE J 1703 (Society of Automotive Engineers), which applies internationally. This regulation specifies criteria such as boiling point, chemical neutrality, water compatibility, rubber swelling, corrosion and lubricity, which must neither be exceeded nor fallen short of. A very important point in the SAE regulation is the requirement of miscibility and compatibility of brake fluids among one other. The safety standard FMVSS No. 116 from the American Department of Transport for glycol based brake fluids DOT 3 and DOT 4 based on SAE regulation J1703 is even more significant. The requirements defined for DOT 5 apply to silicone-based brake fluids.
Standard DOT 5.1 also deals with conventional brake fluids, which meet these requirements and were developed for particularly high demands on safety. As DOT 5.1 brake fluids do not contain any silicone, they are miscible with others of the same specification (DOT 3, DOT 4 and DOT 5.1).
1. BF standards
- SAE J 1703/1704
- ISO 4925
- FMVSS 116
- JIS K 2233
2. Explanation of standard designations
- SAE: Society of Automotive Engineers
- FMVSS: Federal Motor Vehicle Safety Standard
- DOT: Department of Transportation
- ISO: International Standardization Organization
- JIS: Japanese Industrial Standard
Brake fluid specifications should be used for a specific vehicle or brake system, depending upon specification and approval of the vehicle or brake system manufacturer. In order to prevent misapplication, it is recommended to seek the vehicle manufacturer’s advice. Classification ISO 4925 class 6 represents a new development, which has a low viscosity even at the lowest temperatures. This permits a higher degree of safety and quicker brake response in vehicles equipped with ABS, ASR and ESP/ DSC systems.
Glycol-based brake fluids are hygroscopic and even accumulate water when merely exposed to the atmosphere. This property assures that water constituents will disperse in the brake fluid. At the same time, no isolated water spots can occur which freeze at 0 °C and boil at 100 °C. However, even the slightest water content will lower the boiling point of the brake fluid.
Brake fluid change
It is, therefore, recommended to change the brake fluid every year but not later than every 2 years, irrespective of the mileage. All references to health hazards when handling brake fluids must be strictly observed. Brake fluid has a highly solvent and discolouring nature. Because of this, any paintwork, shoes or clothes contaminated by brake fluid must be rinsed immediately with copious quantities of water.