The Trainer #143 - The breakdown of brake fluid characteristics

Welcome to another episode of “The Trainer,” brought to you by Autel. In this episode (#143), Motor Age Technical Editor, Brandon Steckler discussed the important role brake fluid truly has in our vehicle’s brake systems. These systems include the base brake hydraulic system, the ABS/Traction control system, and brake mitigation provided by the Advanced Driver Assist Systems (ADAS) that exist in most vehicle platforms today. 

Like any hydraulic system, the automotive braking system relies on the displacement of brake fluid for component actuation. Physics works as it does because fluid cannot be compressed. With that in mind, any movement of the piston (at the master cylinder) will create a similar displacement at the actuator. Of course, the amount of output vs input is carefully engineered into the design of the brake hydraulic system and the size of the pistons in both the master cylinder and the actuators (Brake calipers and wheel cylinders) at all four corners of the vehicle.  

Brake fluid DOT rating exists to ensure several criteria. These include designating the brake fluid’s ability to remain “a fluid” at both low and high temperatures. Meaning, it must also resist boiling. As brake actuator pistons are displaced by the master cylinder piston, though the brake fluid, the linings of the brake pads and/or shoes come into contact with the brake rotor and/or drum surfaces. The heat given off from the friction between them is the result of mechanical/rotational energy being converted into heat energy. This necessary transfer of energy is what allows the vehicle to slow or stop.  

The heat generated cannot exceed the temperature rating of the fluid or else it will boil. Since brake fluid is hygroscopic (ability to absorb water), as it ages boils it takes on water. Of course, water boils at only 212 degrees F. When a fluid boils it becomes a gas and gasses are compressible. Meaning, that the functionality of the hydraulic system will not exist and instead of displacing actuators, the application of the brake pedal will simply compress the gasses.  

The older DOT ratings are basic and don’t possess the characteristics necessary for the proper operation of today’s autonomous braking systems. The manufacturers' standards are far more stringent.  

Join Brandon as he demonstrates the Autel Maxi-TPMS TS900 and its ability to easily actuate the solenoid valves in the ABS hydraulic unit. This allows any technician the reassurance that a fluid exchange not only replaces the fluid in the master cylinder, hydraulic brake lines, and actuators. But ensures the ABS hydraulic unit is free from any spent/contaminated old fluid.