Brake Bleeding

Brake bleeding is something that no sane person finds enjoyable. It can be messy and often involves bodily contortionism to access the bleeder screws. However, properly bled brakes are one of the most vital services you can perform on your car.

When & Why
Many brake problems can be traced to air or contamination of the hydraulic fluid. Leaks at fittings where lines and hoses connect to the master cylinder, calipers and/or wheel cylinders can allow air to enter the system, as can rusted-through brakelines. Also, air is introduced whenever a brake-system component other than the friction material (pads or shoes) is replaced. Air can also appear after moisture gets into the fluid and boils, giving off steam. Two signs of air in the fluid are a noticeable decrease in brake performance and a "spongy" pedal that quickly and nearly effortlessly goes to the floorboard (air compresses easier than fluid).

Fluid Facts
Although a properly functioning brake system is closed to outside corruption, brake fluid still degrades over time. Fluid must withstand heat that can exceed 650 degrees F at the wheel without boiling and also remain in a fluid state during the coldest winters. Furthermore, brake fluid lubricates and fights corrosion on internal parts.


Most brake fluid is hydroscopic: It absorbs water. On the positive side, any moisture that enters the system—through condensation, through rubber parts such as seals and hoses or through an unsealed bottle of brake fluid—is dispersed throughout the fluid. That way, chances of localized corrosion and fluid freezing are minimized. On the other hand, absorbed moisture lowers the fluid's boiling point, raises its freezing point and degrades its anti-corrosion additives. As little as 3% moisture can decrease fluid's performance by 30% and also negate its anti-corrosives. Additionally, standard DOT 3 brake fluid can only absorb up to 7% moisture, after which water droplets—and braking problems—begin to form. Anti-lock brake systems (ABS) are especially affected by moisture because they can cycle as many as 20 times per second. Spongy pedal feel is a sign of moisture in the brake fluid.


Depending on temperature fluctuations and humidity, a brake system can take on as much as 3% water in as little as eight months. It's usually recommended that the fluid be entirely flushed at least every two years or 24,000 miles.