You may have heard the phrase, but what does it mean to “bleed your brakes”? Certainly, you’re not looking to squeeze them for every penny they have. Nor are you empathetically feeling their pain. And you’re not trying to drain the lifeblood from them, right? Well, maybe a little.
Bleeding the brakes is a common procedure aimed at removing air from the hydraulic brake system on your vehicle. The process involves opening a valve to allow air (and some brake fluid) to escape under pressure.
But why is air in your brake system to begin with? Why would your brakes need to be bled?
How your brakes work
To understand why it might be necessary to have a technician bleed your brakes, it helps to know the very basics of brakes, the components, and what they do.
When you step on the brake pedal, a hydraulic pump (the master cylinder) exerts pressure against hydraulic oil (the brake fluid) inside a network of tubes and hoses (the brake lines). Since a liquid does not compress easily, force applied to the fluid at one end of a brake line is felt at the other end. The effect is similar to what would happen if you held onto one end of a long pole or stick. If you were to push the stick on your end, a person holding the other end would feel the pressure.
That is what happens with your brakes. You press the pedal, and pressure is applied via the master cylinder, through the brake lines, to a clamping mechanism on the other end called a caliper. The caliper suspends a pair of brake pads on either side of a heavy metal disc (the brake rotor) that is attached to – and spins with – the wheel and tire. The caliper squeezes the rotor and causes friction to slow and stop your car. On a vehicle equipped with a disc brake system (most vehicles today), there is a rotor, caliper, and pair of brake pads for each wheel.
Where the air comes in
There are a number of things that can go wrong with your brakes over time. Your brake pads will wear out – they’re supposed to. Your rotors will wear down too. They’re not supposed to, but they do. Calipers can corrode and get stuck. Brake lines can rust. The master cylinder can fail. Since your brakes are essential to your safety and the safety of your passengers (not to mention everyone else on or near the road), it is important to have your brakes serviced regularly.
Part of that service can include bleeding the brakes. But how does air get in the system, and what is the problem if it does?
Well, air can enter a hydraulic brake system in a few different ways. First, any time the system has to be “opened”, meaning a connector is detached allowing brake fluid to escape, air can get in. If a new brake line or caliper is installed, the air inside the new component(s) needs to be bled out. That is one reason it is not wise to choose cheap brake service where only partial work is done or is performed by someone less than qualified (sorry Uncle Larry). A complete brake job comprises not only replacing brake pads, but servicing the rotors, calipers, and other system components – including bleeding the brakes when necessary.
Another way air makes its way into the brake system is when your brake pads become excessively worn – when you wait too long to get your brake pads changed. Your brake pads will tell you when it is time for a change. You might notice an indicator light on the dashboard. Otherwise, you will hear a shrill squealing sound when you step on the brakes as the brake pad wear indicators contact the rotor and sing out a song to remind you to have them changed. Wait any longer and the thin pads will allow the brake fluid level to drop considerably and let air get in. (If you want your brakes to last longer, ask about ceramic vs. semi-metallic brake pads).
A fluid leak somewhere in the brake system can also allow air to enter. Leaks occur when brake lines are corroded or damaged. A bad caliper or wheel cylinder can also be the culprit.
Often, air in the brake lines is a result of heat. High temperatures result if you tend to ride your brakes or brake aggressively. Actually, your brakes get pretty hot anyway, but those habits make matters worse. When brake system components get hot, the fluid boils. Brake fluid is hygroscopic, it absorbs moisture. When the fluid boils, the moisture turns to vapor – the equivalent of air in the lines.
What is the problem with air in the lines? Hydraulic brake fluid will not compress in your brake system. It transfers the pressure from the pedal to the calipers. Air (or vapor), on the other hand, compresses easily. It gives. It squishes. It absorbs the force. So, when you apply pressure at the pedal, instead of fluid causing the calipers to squeeze, air simply compresses in its space and prevents force from being applied through the lines.
What you will feel on your end is a soft, squishy pedal or one that sinks to the floor.
How are brakes bled?
To remove the air in your brake system, a technician will systematically open up a bleeder screw (valve) on each caliper (or on the wheel cylinders in a drum brake system) and drain off the air (and some fluid). This is done with assistance from someone inside the car pumping the brake pedal, or from a power bleeder connected to the master cylinder’s reservoir (this device pressurizes the system while adding fresh fluid). Care must be exercised so as to avoid sucking more air into the system. If a two-person approach is taken, actions by both technicians must be coordinated so that the bleeder screw is never opened when the pedal is released. The master cylinder reservoir must not be allowed to run dry either.
Bleeding the brakes is not necessarily a highly-complex procedure. But it is an essential one. It also presents some risks. First of all, the vehicle must be raised off of the ground and supported on jack stands or a vehicle lift. Careless operation of the bleeder screw can result in damage to the caliper. Brake fluid is relatively caustic and can cause burns to the skin and eyes, and to the paint and other surfaces of your car. More importantly, if done incorrectly, your brakes will not function properly.
Attention to your car’s brake fluid is among the most essential elements of brake maintenance. Make sure your brakes are in proper working order and take it in for brake repair at the first signs that it is not.
Columbia Auto Care & Car Wash | Author: Mike Ales | Copyright
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