Like the blood in a human being, brake fluid performs a vital function in a hydraulic brake system. Improper handling of the fluid can cause the brake system to fail, leading to a serious accident.
Brake Fluid Characteristic Overview
Brake fluid is used in the hydraulic circuits of brake and clutch systems. Force applied to a vehicle’s brake or clutch pedal is transmitted to the brakes or clutch by fluid that fills the pipes in the hydraulic circuits. In contrast to engine oil and automatic transmission fluid, brake fluid is a non-mineral substance; it is not a type of oil. The main requirements of brake fluid are as follows:
High Boiling Point
When a vehicle’s brakes are applied frequently on a downhill road, heat generated by friction in the brakes is transmitted to the fluid via the brake calipers. This heat causes moisture in the fluid to boil, thus creating vapor in the system. When the brake pedal is depressed, the vapor is compressed without transmitting the pedal force to the brakes. As a result, the brakes stop functioning. To prevent this problem which is known as vapor locking, brake fluid must have a high boiling point.
Low Viscosity
Brake fluid must flow smoothly over a wide temperature range and be capable of operating normally in very cold conditions.
Compatibility with Rubber and Metal Components
Brake fluid must have a chemical composition that does not attack the rubber and metal components used in the brake system.
Brake Fluid Characteristics
Brake fluid is highly hygroscopic, i.e., it absorbs moisture from the atmosphere. Moisture in the brake fluid lowers the fluid’s boiling point and makes the system prone to vapor locking. In addition, moisture in the fluid causes metal components to corrode. In extremely cold conditions, moisture in the brake fluid can freeze and prevent the brakes from functioning.
To prevent contact with moisture, brake fluid containers must be sealed immediately after use.
Fluid in a vehicle’s brake system gradually absorbs moisture. Irrespective of the distance driven, therefore, the brake fluid replace regularly.
Brake fluid attacks plastic and painted surfaces. If fluid accidentally spills onto vehicle paintwork or plastic components, it must be washed off immediately.
Brake Fluid Grades
Brake fluids are manufactured to conform with the United States Department of Transportation (DOT) Federal Motor Vehicle Safety Standard (FMVSS). There are three grades: DOT 3, DOT 4, and DOT 5. Each DOT grade specifies the required boiling point, viscosity, and other properties. The higher the DOT number, the higher the boiling point.
Brake fluid conforming to the DOT 3 or DOT 4 standard should be used in vehicles.
Never use DOT 5 brake fluid in Honda vehicles. The basic chemical composition of DOT 5 fluid is different from that of DOT 3 and DOT 4 fluids. If used in a regular brake system, DOT 5 fluid can damage rubber components and cause leakage. Also, DOT 5 fluid is not hygroscopic. If water enters the brake system, therefore, it remains in its original state without being absorbed by the fluid. As a result, the risk of vapor locking is greatly increased. (For reference, the minimum dry boiling point of DOT 3 brake fluid is 205 degree Celsius. The boiling point of water is, of course, 100 degree Celsius.)
This article provides an overview of brake fluid characteristics.