How To Flush and Replace Brake Fluid

After working as a General Motors and Automotive Service Excellence (ASE) certified master technician, I transferred those skills into a new career — teaching! I spent 25 years instructing the next generation of automotive service professionals at a local vocational high school. It was the hardest yet absolutely the best job I ever had.

Brakes and braking systems were a huge part of my curriculum. Here’s a story I told my students: Once while working in a repair shop, I had to rebuild a set of brake wheel cylinders. The insides of the cylinders were filled with rust and muck. The debris etched the cylinder walls, damaging the inner cup seals. This caused them to leak, sending the brake pedal to the floor.

All those years ago, I didn’t understand brake fluid is extremely hygroscopic, meaning it can absorb and hold moisture.

How does moisture enter a sealed brake system?

A small vent in the master cylinder prevents brake system vapor lock, which could seriously affect braking performance. The vent allows air into the reservoir as the brake fluid level drops when stepping on the brake pedal, then exits when stepping off the pedal. Air entering the master cylinder contains moisture, dust and dirt that contaminates the fluid.

Moisture in brake fluid causes metal brake parts to rust and corrode from the inside out (aka a rotted-through brake line). It also causes rubber parts to swell, deteriorate and weaken. Rust and corrosion floating in the brake system can clog up and ruin expensive anti-lock brake system (ABS) pumps and valves.

Can I flush brake fluid myself?

Yes. Although enlisting a helper makes flushing brake fluid easier, if you’re comfortable safely jacking up a car and taking off wheels, you can flush the entire brake fluid system yourself.

Flushing brake fluid sort of like bleeding brake fluid. Bleeding the brakes removes air bubbles from the system. Air bubbles compress, causing a spongy or soft brake pedal or brake pedal fade. Flushing brake fluid removes all the old fluid and replaces it with new. Here’s how to do it.

Special thanks to the administration and automotive technology program at North Montco Technical Career Center in Lansdale, Pennsylvania, for assistance with this article.