It’s probably a safe bet that street rodders tend to pay above-average attention to the maintenance of all their cars. But it’s probably an equally safe bet that few of us consider brake fluid as something that should be replaced on a regular basis, however let us explain why it should be.

Brake fluid can easily become contaminated, and you don’t have to drive your car every day for it to happen. Moisture from the atmosphere can be absorbed by the brake system in a number of ways and it is a common problem. So much so that a recent National Highway Traffic Safety Administration (NHTSA) survey found that the brake fluid in 20 percent of 1,720 vehicles sampled contained 5 percent or more water.

Testing Brake Fluid

The condition of your car’s brake fluid is hard to determine visually, unless it’s extremely dark or has obvious signs of rust. However there are a number of affordable methods to check its condition:

An optical refractometer is a device that measures the amount of water in any water-soluble fluid. They’re used to examine everything from wine to maple syrup and there are versions for brake fluid as well as that will also test antifreeze levels. Prices range form as little as $60-$100 or more.

Many parts houses carry inexpensive chemical test strips that will show the presence of copper in brake fluid. When copper levels rise above the recommended level it’s an indication that the rust inhibitors in the fluid are no longer effective.

Electronic brake fluid testers measure the amount of water in the fluid. Surprisingly affordable, some sell for under $30. The test takes only about a minute and is quite accurate.

Brake Fluid Tips From AMSOIL Inc.

Brake Fluid Failure

There are two basic failure modes for brake fluid: It can boil, and it can cease to provide adequate lubrication and corrosion protection. Both are the result of contamination, usually with water or petroleum products.

DOT 3 and DOT 4 glycol brake fluids are hygroscopic; they absorb water easily and hold it in suspension, much like antifreeze. The second the cap is removed from a bottle of brake fluid, the brake fluid begins absorbing water from the air. And while brake fluid is kept in a closed system,

moisture seeps into the brake system continuously through the various seals and microscopic pores of the flexible brake lines. Moisture absorption through the brake system accelerates as vehicles age, and there’s almost no limit to how much water brake fluid can absorb. Water lowers the boiling point of brake fluid, which will eventually lead to failure. After two years of service, the average boiling point of brake fluid will have dropped below the minimum federal requirements.

High Boiling Points Prevent Failure

As the brake system heats up, brake fluids with low boiling points begin to vaporize. The brake pedal must travel further to apply the same amount of force on the brakes, causing a spongy feeling. If enough of the brake fluid is vaporized, brake system failure may occur.

Brake fluid boiling points are measured on two separate scales: (1) dry equilibrium reflux boiling point (ERBP), the boiling point of new, freshly opened unused fluid; and (2) wet ERBP, the boiling point of a brake fluid after it has absorbed 3 percent water.

The Department of Transportation (DOT) has set minimum standard requirements to ensure brake fluids will provide adequate protection. The minimum dry ERBP for DOT 3 brake fluid is no less than 205 degrees C (401 degrees F), the minimum wet ERBP is 140 degrees C (284 degrees F). It only takes 4 percent water contamination to bring a DOT 3 fluid’s boiling point down to the federal limit.

The minimum dry ERBP for DOT 4 brake fluid is 230 degrees C (446 degrees F), the minimum wet ERBP is 155 degrees C (311 degrees F). Their higher boiling points make DOT 4 fluids appropriate for high-performance cars and motorcycles, and for vehicles used for towing. Because brake fluids are hygroscopic, the wet boiling point is of much greater concern than the dry boiling point.

AMSOIL Inc. Brake Fluids