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Stopping your car with juice

Hydraulic brake fluid is here to stay – for now. Help techs know why checking fluid health is critical to vehicle safety.
Thursday, November 1, 2007 - 00:00

Hydraulic brake fluid is here to stay – for now. Help techs know why checking fluid health is critical to vehicle safety.

Hydraulic brake actuation has been with us for some time. It caught on in the mid-1920s when upstart Walter Chrysler made it standard on affordable cars, and by 1939 even Henry Ford had abandoned "the safety of steel from pedal to wheel" in favor of hydraulics.

We will soon see solenoid- and/or motor-driven calipers start to replace hydraulics. But even if brake fluid eventually becomes as rare as points and condensers are today, that day is far away. Caution techs that they will be servicing hydraulic brakes for some time.

One problem motorists may experience resulting from hydraulic problems is pull. (Ironically, one reason hydraulic brakes caught on was because mechanical brakes had an even greater tendency to pull as cables and rods stretched unevenly.) Hydraulic woes can lead to hard or soft pedals. There's also a danger of losing brake fluid. Motorists neglect their hydraulic systems at their peril.

Brake fluid maintenance

Henry Ford had a point when he was leery of leaking "brake liquid." Brake fluid loss can lead to hydraulic failure. Granted, dual master cylinders were mandated in 1967, splitting braking circuits into either front/rear or two diagonal systems, so leaks aren't as catastrophic as before. But they're still very dangerous.

External forces, such as abrasion or impact damage to tubing/hoses, can cause hydraulic leaks. Internal problems, such as caliper-seal or piston-cup failure or tubing rust-through, also may allow brake fluid to escape. One way techs can prevent rust on the inside of tubing is to get rid of old brake fluid — which also provides other benefits.

Some vehicle manufacturers prescribe periodically flushing out old brake fluid and replacing it. A typical interval, often called for by overseas-based automakers, is every two years, regardless of mileage.

Their reasoning is that brake fluid may absorb enough moisture in that time to lower its boiling point and damage internal components' insides. Despite the fact that they might be able to sell more replacement parts if everybody kept their old brake fluid, aftermarket brake manufacturers echo this recommendation.

Detroit, on the other hand, has traditionally shied away from specifying such an interval. However, nobody in Motown endorses the idea of hanging onto brake fluid that's demonstrably deteriorated.

Location is crucial

Does this mean techs should periodically open up master cylinder reservoirs and test fluid samples? Not really. Sample testing is a good idea, but the reservoir isn't the right place to get a sample. Instead, have them use caliper bleeders.

Differences of opinion regarding brake fluid deterioration and maintenance have prompted the National Highway Traffic Safety Administration (NHTSA) to commission the National Institute of Standards and Technology (NIST) to study these topics in 1998. NIST concluded that the rate of deterioration varies among vehicles based on driving habits and environmental conditions.

But no matter how fast it deteriorates, the fluid, as it becomes corrosive, first attacks copper, a material in the inner lining of steel brake tubing. As a result, an early sign of brake fluid breakdown is the presence of copper in the fluid.

NIST also found that brake fluid goes bad within caliper bores first, because the heat generated while stopping breaks down corrosion inhibitors in brake fluid. As a result, the fluid in some vehicles could be extremely corrosive in and near the calipers, but still appear fine in the reservoir.

This is especially possible in cars and trucks without antilock brake systems (ABS) because the brake fluid doesn't move around in these vehicles. But ABS and stability/traction control systems, with their Gatling gun-like removal and reapplication of brake fluid pressure, tend to force some movement of brake fluid throughout the hydraulic system. The results can be costly once that worn out brake fluid lands in expensive components, such as the ABS hydraulic control units (HCUs) that have small, precisely machined internal parts.

So when should techs check and change brake fluid? The Automotive Maintenance and Repair Association (AMRA) recommends checking brake fluid copper content at the vehicle maker's recommended brake inspection intervals. AMRA calls for replacing brake fluid when the copper content reaches 200 parts per million (ppm) or more.

Checking for copper contamination is simple. Use test strips – similar to the litmus paper used in high school chemistry class or to test chemical concentration in a pool. Strips are available to specifically determine brake fluid copper content.

Paul Zangari is a freelance writer specializing in technical automotive subjects. He also is the host of a weekly radio show in Providence, R.I., called "Drive-Thru Radio" on station WPRO-AM.

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