Brake fluid maintenance services can be the toughest item to sell. Oil can become dark and transmission fluid can smell funny, but brake fluid in the reservoir can look clear and still be in need of replacement.

Brake fluid can become contaminated by water or outside debris, but the main damage to brake fluid over time can’t be seen. Corrosion inhibitors, pH stabilizers and antioxidants are added to brake fluid to improve the long-term corrosion protection of brake systems. Over time, these additives ­become depleted and ­destroyed, leaving the internal parts of the brake system vulnerable to corrosion.

The rate of depletion is ­affected by many factors. One study found the rate of depletion is fastest at the wheels. This is where the fluid is exposed to the highest ­degree of heat, which causes the corrosion inhibitors to breakdown. The same study found that ABS systems are also a hot spot for ­corrosion.

What does the corrosion do? Without the corrosion ­inhibitors and pH buffers, the fluid can pit iron and steel components. It can also ­corrode the copper brazing used to seal the brake lines. Since it is a closed system, the particles will float around the system and stick to and damage other components. Degraded fluid can then bypass master cylinder seals and cause ABS valves to stick.

How do you sell something you can’t see? The answer is to measure the moisture and copper content of the fluid. Test strips can be used to measure the amount of copper concentration in the brake fluid. The amount of copper in the system is an indicator of the health of the additives in the brake fluid.

The test is simple and straightforward. Simply dip the strip in the brake fluid in the reservoir for one second. In 30 to 120 seconds, the ­reaction zone will change colors depending on the condition of the brake fluid. Compare the color of the ­reaction zone to the color key and make the appropriate recommendation.

Test strips, however, are not a replacement for measuring the boiling point of the brake fluid.