Why Do My Brakes Lock Up When Trying to Stop?

Brakes locking up, whether in an emergency or while simply slowing down, can be a frightening experience. Here's what you need to know.

With the advent of the anti-lock braking system (ABS) that automatically “pumps” the brakes during a panic stop or normal stopping on slippery roads, brake lock-up has been virtually eliminated. First offered as an option in the 1970s, ABS has been a standard feature on virtually all new cars built since the early 2010s.

Still, brakes may still lock up occasionally, significantly increasing stopping distance and greatly reducing directional stability. That’s a problem. Here’s what you need to know about brakes locking up on vehicles with or without ABS. Check your owner’s manual or the instrument panel for the yellow ABS indicator light to determine if your vehicle has ABS.

Causes for Brake Lock-Up

Road Conditions

On vehicles without ABS, loss of traction between the tire tread and road surface when stopping on wet or slippery roads causes brake lock-up. Even though you’re pushing on the brake pedal as hard as possible, and your tires are no longer spinning, your tires keep skidding on the slick surface. This happens because the tires have nothing to grip onto to develop the traction needed to stop.

For maximum stopping power on slick surfaces, ABS pumps the brakes for you. This allows your tires to almost, but not quite, skid.

Mechanical Failures

On standard and ABS-equipped vehicles, minor issues such as worn brake pads, excess rust on the rotors (from your vehicle sitting for months) and installing brake pads on one axel that may heat up slower than the pads on the other axel can cause brake lock-up during normal driving conditions. Replacing worn pads (a DIY task) and normal driving should wear away anything but extreme rust build-up. Once the brake pads heat up, you shouldn’t experience any braking problems.

Let’s look at other reasons brakes lock up when they shouldn’t.

Brake Calipers

Non-ABS and ABS: Worn, missing, rusted or broken mounting hardware can cause disc brake calipers to bind, overheating the brake pads and rotors. Overheating leads to premature pad and rotor wear and brake lock-up that can cause your car to pull sharply to one side when stopping. Replacing pads and rotors can be a DIY project.

Hydraulic System

Non-ABS and ABS: A low brake fluid level, using the incorrect brake fluid, or brake friction material saturated with brake fluid from a leaking component can cause brake lock-up. A collapsed brake hose trapping fluid in the caliper will behave the same as a binding caliper. A bad brake system proportional valve delivering equal fluid pressure to the entire brake system can cause the rear wheels to lock-up during heavy braking.

ABS only: Brake fluid contaminated from moisture can damage the ABS pump. A failed ABS pump will cause poor braking performance as well as brake lock-up. Never drive a car if you suspect a brake fluid leak.

Wheel Speed Sensors and Wheel Bearings

ABS only: Wheel speed sensors measure a wheel’s speed via a ring gear on the wheel hub bearing and sends that information to the car’s Engine Control Module (ECM). A worn wheel hub bearing, damaged ring gear or failing speed sensor sending inaccurate data to the ECM may engage the ABS for no apparent reason when stopping, locking up the brakes.

Check your tire pressure. A tire with low pressure will roll at a different speed than the other tires, sending faulty data to the ECM.

ABS Computer

This encompasses the ABS controller/module and or ABS pump and is the ‘brains” of the system. A malfunctioning ABS computer can cause exactly what ABS tries to prevent — locked-up brakes. A problem with the ABS computer or electronics will cause poor braking performance. It will also activate the ABS dash warning lamp, letting you know it’s time to visit your mechanic.

Driver Reaction to Road Conditions

If your brakes lock up while stopping, remain calm!

On non-ABS vehicles, pumping the brake pedal will help the tires maintain traction if the brakes lock up due to a mechanical issue or if skidding on slippery roads.

On ABS-equipped vehicles, holding your foot firmly on the brake pedal should automatically activate the ABS when the computer senses a skid condition. In addition, pressing down hard on the brake pedal may activate the ABS if the system isn’t working properly. Conversely, never pump ABS brakes. This is a sure way to confuse the computer, making it more difficult to stop and steer your vehicle. The ABS is designed to alert you if there are any problems with the system.

Always keep in mind, brakes are one of your car’s most important safety features. If something isn’t working properly, it’s time to see your mechanic.

Robert Lacivita
Bob Lacivita is an award-winning auto technician and career and technical educator and freelance writer who has written about DYI car repairs and vehicle maintenance topics, as well as writing state, federal and organizational foundation grants, and helped design a unique curriculum delivery model that integrates rigorous, relevant academic standards seamlessly into technical/vocational training, for more than 20 years. His work has been featured in Family Handyman, a Reader's Digest book and Classic Bike Rider magazine, among others. Bob and his wife lived through 20 years' worth of DIY home remodeling while parenting two (now grown) boys and now relax by watching their three fabulous granddaughters.