The Correct Brake Fluid Is a Must
Question: I recently needed to add some brake fluid to my car. In the auto parts stores, I noticed that some cans were rated DOT 3, some DOT 4 and others DOT 5. What are these ratings, and does it make a difference which fluid I use?--H.A.
Answer: It is important to use the correct type of brake fluid, though consumers are given much less guidance on making that choice than with motor oils or even transmission fluids.
In most cases, the owner’s manual to your car should tell you exactly which type of brake fluid your car takes. Sometimes the rating is also noted on the top of the brake master cylinder. Be sure to use the recommended fluid.
The DOT rating, which stands for Department of Transportation, refers to what is called the dry boiling point of the brake fluid (the temperature at which the fluid begins to boil). DOT 3 fluid boils at a minimum of 401 degrees; DOT 4 boils at a minimum 446 degrees. In some cases, DOT 3 is used in drum-type brake systems and DOT 4 in disk-type brake systems, but this is not always the case.
These boiling points are ratings set when the brake fluid is completely dry or devoid of moisture. But brake fluid readily absorbs moisture when it is in the brake system, and that moisture can lower the boiling point significantly.
For instance, the boiling point of DOT 3 drops to 284 degrees when wet, that of DOT 4 to 311 degrees. In heavy usage, a disk-brake system can reach 1,000 degrees, and much of this heat is transferred to the brake fluid.
These specifications are very important to the safety of your braking system. If the fluid boils, tiny bubbles form, which lowers the performance of the brake. When you step on the brake pedal, instead of a steady and solid pedal, the action becomes spongy. Moreover, it may take greater pedal effort to stop the car when you have water in the brake fluid, all other things being equal.
Most DOT 3 and 4 brake fluids are made of glycol, which is a member of the alcohol family. Glycol readily absorbs water, so it is not uncommon for older brake systems to have water-contaminated brake fluid. That is one reason why the manufacturers of many brake components recommend changing the brake fluid every two years. Most car manufacturers do not make this recommendation.
DOT 5 brake fluid attempts to solve the problem of water contamination by using a base of silicon, which does not absorb water. You don’t really want to use DOT 5 in a system that has used DOT 3 or 4, because it could form a sticky sludge. If you want to convert your system from DOT 4 to DOT 5 fluid, you should, according to some experts, replace every rubber seal and clean out the system with solvent, which is more work than many motorists want to take on.
