By Jason Eastman, GMR Member and owner, Bullfrog Auto, Milton
How often should I change my brake fluid?
It comes from member Bob Costella. His Porsche dealer told him every two years for his 911 but his Audi dealer has never brought up the subject when he has taken in his A5 for service. (And I would add our manual for our 2019 VW Tiguan demands every three years. --Editor).
Here's what Jason has to say:
Brake fluid is to be changed every 2-3 years no matter the car brand. More frequently if the car is tracked or driven aggressively.
The reason for the fluid change is moisture. Most brake fluids are hygroscopic, meaning they will absorb moisture when exposed to air. The main enemy of brake fluid is moisture which can be absorbed through brake lines or the brake fluid reservoir even when the vehicle is not driven. This moisture does three things.
- It lowers the dry boiling point of the brake fluid
- It causes contamination that can lead to corrosion of the internal components.
- It can freeze in cold weather resulting in blockages and loss of braking ability (very rare).
All brake fluid manufacturers have two boiling points, dry and wet. The dry boil point is the point at which the fluid will boil when fresh. A car with new brake fluid will have a dry boil point (e.g. 500 degrees) set by the manufacturer.
A car with older brake fluid that has absorbed moisture will have a wet boil point that is lower (e.g. 350 degrees). The wet boil point is lower due to moisture accumulated over time in the fluid. When the brakes are used, they generate heat.
This heat radiates into the brake calipers where the fluid is pushing onto the brake pistons. The fluid in the calipers absorbs the heat. If the fluid gets to the wet or dry boiling point, it will boil. The boiling causes air bubbles to form in the brake system.
Brake fluid does not compress, but air does. As a result, the air in the fluid causes a mushy brake pedal and/or brake fade (i.e. loss of braking ability). By flushing out the fluid, the boiling point is restored to the dry point or close to the dry point.
Thanks Jason! If you have a car question you'd like shared with our membership, just reply to this email and we'll send it to Jason, who with his wife Sue (GMR Secretary) owns Bullfrog Auto in Milton.