Car brakes may just be the most important safety feature on any car. Maintaining them is crucial, but a lot of people don’t understand how brakes work or what to look for to know whether they’re operating properly. In addition to the obvious safety reasons, maintaining your brakes will help with resale value.

At a basic level, cars like your BMW use what are called “friction” brakes. These brakes use the contact between two types of material to bring the car to a halt. A friction brake may be either a disc brake or a drum brake.

In drum brakes (usually only on older cars), brake shoes are inside a rotating metal drum. When you use the brakes, the shoes push outward against the drum. The friction between the brake shoes and the inner surface of the drum slows the car down. Drum brakes tend to be less efficient than disc brakes. Brake drums are usually made of cast iron or aluminum, while brake shoes are often made of steel-lined friction material.

Spongy brakes are generally the result of air, gas or water inside the hydraulic system. Car brakes use a hydraulic fluid that transfers force through high-pressure brake lines. If you’ve got something other than brake fluid inside those lines, the brake fluid is less efficient in transferring force. That leads to a spongy feel at the brake pedal.

The most common causes of this trouble stem from the brake fluid itself. If there’s any water in the brake line, heat from the brakes can vaporize the water, resulting in that spongy feeling. Or in some cases, the brake fluid can actually boil if the brakes get too hot.

The easiest thing to do about spongy brakes is to change the fluid. In most cases, that will get the brakes back to operating normally, unless there’s another problem with the brakes. In general, it is wise to have your brake fluid changed periodically. Besides causing spongy brakes, water in the brake fluid causes corrosion that can ruin calipers and brake cylinders. Brake fluid changes are relatively cheap, so you should have it done every year or two. That’s all part of proper maintenance for your car, as covered in Top 10 Car Care Tips.

ABS stands for anti-lock brakes, a braking system that has been standard on all new cars, trucks and SUVs sold in the United States since 2013.

An anti-lock brake system has sensors on the wheels that report to a computer when a particular brake locks up or begins to skid. If a lockup occurs, the computer takes over and orders the system to begin pulsing the brake. If you’ve been driving for a while, you may remember the old advice to “pump the brakes” to prevent a loss of control. ABS means you no longer need to worry about that. In an anti-lock system, the pulsing is very quick: roughly 12 times per second. Every system works differently, but generally the pulsing occurs so quickly that you could not really detect it from the feel of the brake pedal.

One of the factors that you need to look for in a car brake is the peak force. This refers to the maximum force a brake can apply. It determines how fast a brake can slow down a car. Another important factor is fade. When car brakes heat up (and they heat up and cool down constantly when you’re driving), they lose friction and result in weaker braking force. This is known as brake fade.

How you use your car determines what kind of car brakes you need. If you drive fast and use your brakes often, you need car brakes with superior heat dissipation. Because car brakes also wear out, you need to check them occasionally. Worn-out car brakes can cause an accident, so it makes sense to check and service them regularly.