Updated: Mar 26
Needless to say, brake fluid is the most critical aspect of your braking system. Most vehicles on the road today use hydraulic braking. Other types of braking systems are pneumatic and mechanical braking. For this article, we'll be discussing what's in your daily driver...hydraulic braking.
According to wikipedia.org, hydraulic braking was first patented and used around the 1920's by Malcom Loughead (aka Lockheed). This technology was started off in racing vehicles and tractors. It eventually evolved into what we know today has hydraulic brakes.
Hydraulic brakes working on the basic principle of fluid pressure. Fluid under pressure essentially becomes 'heavier'. If it gets heavy enough it can do work. Such as, move pistons and/or valves. Brake fluid is the medium for which your brake system operates. Just as you need blood for your heart to pump, your brakes need brake fluid.
Brake fluid is stored in the fluid reservoir (which is almost always part of the master cylinder). The master cylinder is the 'heart' of your brakes. It's what pumps the brake fluid through the vehicle's brake circulatory system. Every time you step on the brakes, the master cylinder pumps and pressurizes brake fluid. When brake fluid becomes pressurized, it's pressure remains constant throughout the entire system: (from your master cylinder to the brake calipers). Brake fluid pressure needs to remain constant in order for the brake fluid to move those pistons and/or valves. Remember how we said brake fluid basically becomes 'heavier' the more it's pressurized? When the brake fluid is pressuried, the weight of fluid will overcome the frictional resistance of the brake caliper pistons and begin to move them hydraulically. Think of spraying a pile of leaves with a garden hose, you have to place your thumb over the hose to provide enough pressure to move the leaves.
Everyone has heard of DOT 3 and DOT 4 brake fluid. The 'DOT' stands for Department Of Transportation. Both fluids are made of glycol ether and borate ester. The key differences between the two are water absorbtion and boiling points. DOT 3 fluid absorbs less water whereas DOT 4 is better suited for higher temperatures. Consult your owner's manual or OEM service manual to find out what rating brake fluid your vehicle uses.
Yes, brake fluid gets dirty. The #1 reason brake fluid should be changed annually is due to moisture retention. Brake fluid absorbs moisture in the air; this is called 'hygroscopic'. This moisture collects in the hydraulic system and can lead to internal component failure such as leaking lines, seals and/or master cylinder failure. Dirt particles and heat also play a role in brake fluid contamination. Rubber components such as brake hoses can corrode internally and float through the hydraulic system causing unwanted blockages. Heat generated from excessive braking or by the activation of the Anitlock Brake System help break down fluid integrity.
We hope this article has been insightful and has helped shed light on what brake fluid is and why it's important to change it. Happy braking!