Why You Should Never Put WD-40 On Your Brakes And Rotors
Of all the uses for WD-40, one it is quickest and best at is quieting squeaky mechanisms like door and gate hinges. This makes it a tempting proposition to want to spray the versatile product on all things noisy and bothersome. It's common to hear squeaking from your car's brakes, which might make you wonder if spraying WD-40 on noisy car brakes will make that annoying squeal go away. But while that familiar blue and gold can with the red cap and straw has a lot of useful functions when it comes to auto maintenance including helping to free frozen door lock cylinders, using it to quiet noisy brakes is unwise, unnecessary, and not recommended.
To know why, it helps to first understand what makes your brakes squeak in the first place. Often, the noise occurs after dew collects in the space between the pads and rotors overnight and traps dirt and other particles. A thin layer of rust forms, and when you apply the brakes in the morning, the pads squeeze the rust and other particles against the rotors, causing a squealing sound.
Brake squeal can also be a sign of worn brake pads
If the issue is simply one of condensation, parking in a garage or other climate-controlled area overnight should eliminate the problem, and gently applying the brakes early in your driving day should make the noise less prominent or go away altogether. Under this circumstance, applying WD-40 might help displace the water and quiet your brakes, but unless you're going to do this every morning it won't really be a useful tactic.
Squeaky brakes can also be a signal that your brake pads are nearing the end of their life and need replacement, making the noise an important safety warning. Spraying WD-40 on your brakes can also damage the rubber seals on the caliper seals and brake lines, causing hydraulic fluid leaks and other potentially dangerous failures of your brake system. Sometimes the source of brake noise is a stuck drum (many vehicles have drum brakes on the rear wheels).
If your drum brakes are sticking, use a brake lubricant instead of WD-40, and apply it to the backing plate where the brake shoe sits and not to the front brake rotors. Applying brake lubricant between disc rotors and pads will severely compromise disc brake performance and could lead to a crash.