Written By: David Zenlea
You hurtle down the long straight, hit the brake pedal, and feel it sink sickeningly as you fly past the turn-in cone. A day at the track, and maybe your car, ruined. Now's the wrong time to ask: When did you last flush your brake fluid?
The hardworking hydraulic messenger between your foot and the brake pads usually does its job without complaint. In a typical street-driven car, brake fluid lasts for a couple of years. But it has vulnerabilities that can bite on track. First, it chemically changes under the extreme heat generated by repeated hard stops. Just like tires, brake fluid should only go through so many heat cycles. The fluid's also hygroscopic, a fancy way of saying it attracts and absorbs water from the atmosphere. Water lowers the point at which the fluid boils. Boiling fluid releases gas. If there's gas in the brake lines, the pedal will sink to the floor. You have no brakes.
Brake fluid's life span depends on what you drive, how you drive, and even where you drive. A powerful, heavy C63 AMG in hot and humid Florida could cook its fluid in a single track day. A Miata in cool and dry Iowa may last much longer. You certainly want to bleed the brakes every time fluid boils. Even if the reservoir looks clean, what's in the calipers could have broken down and trapped gas bubbles.
Your car and your driving will also determine what to pour. Standard DOT 3 or 4 fluid suffices for many. If you're repeatedly experiencing brake fade, splurge on types with higher wet-boiling points, like ATE Typ 200 (374 °F) or Castrol React SRF Racing (518 degrees). But bear in mind, if fluid's failing, other brake components may not be far behind—in extreme cases, the higher temperatures sustained by the performance fluid can actually finish them off. Stay away from DOT 5 fluids; they have high boiling points but don't mix well with other varieties.
You can't buy Brembos in a bottle, but good fluid that's properly maintained can be just the thing that saves your car or life, even. A brake-fluid flush should therefore be one of your rituals. Suck the old fluid from the reservoir (careful, it's corrosive). Top off with fresh stuff—not from an opened bottle that's been absorbing water on a garage shelf—as you bleed each caliper, starting with the one farthest from the master cylinder. Since you're down there anyway, inspect for uneven pad wear, pitted rotors, and frayed or cracked lines. Better to notice a problem now than at the end of that long straight.