Brakes turn the energy of motion into heat. In other words, when you want to stop the friction between the brake materials, and the brake rotors/drums (more on this later) creates heat. The faster you go the more heat the brakes create for you to stop.
There are two types of brake systems, shoes and pads. Shoes are the old system, and they are rapidly being replaced with pads. Many years ago the government mandated that all cars have front brake pads. Most of the cars I work on have 4 wheel brake pads.
When you press the brake pedal you push brake fluid from the master cylinder through the brake lines into the wheel cylinders. The pressure of the fluid in the wheel cylinders pushes two plungers outward which force the brake shoes to make contact with the inside of the brake drum. The pressure/friction of the shoes on the drum causes your car to stop. One of the biggest problems with this system is that the shoes don’t always make 100% contact with the brake drum. When you stop braking springs retract the shoes and remove them from touching the brake drum.
Brake Shoes and Drums
These rarely need replacing. The brake shoes rub on the inside of the brake drum. When the brakes are inspected, we measure the inside diameter of the drum. If the diameter is too large (there is a federal limit which is different for every model) then we need to replace the drum. If the drum is not round, you will feel the brake pedal pulsate, and we need to resurface the brake drum. If the inside surface is flat, we do not need to resurface it when re replace the brake shoes.
When you brake you push brake fluid from the brake master cylinder through brake lines into the caliper. In the caliper, the brake fluid pushes a piston outward forcing the ‘inside’ brake pad onto the rotor. This is a litter difficult to explain, so please bear with me… “For every force there is an equal and opposite force.” When the brake pad pushes against the rotor, the caliper is forced back, causing the ‘outside’ brake pad to make contact with the outside surface of the brake rotor. This is called a sliding caliper. Ideally, both pads exert the same force on the brake rotor. A few manufacturers don’t use sliding calipers. These manufacturers use a brake caliper with pistons on the inside and on the outside of the brake rotor.
Brake Caliper with One Piston
Brake caliper with two sets of pistons. One set for the inside pad, and one set for the outside pad
If a sliding caliper cannot slide, one pad is substantially more worn than the other pad, and you need a new caliper. Another problem with brake calipers is that the piston does not retract when you stop braking. This is also clear by noticing one brake pad being substantially more worn than the other brake pad. When this happens, you need to replace the brake caliper.
Since it is important that the front left brakes and the front right brakes work exert the same forces, when one caliper is bad, it is generally recommended that both calipers be replaced.
Brake pads are simpler than brake shoes, and they work significantly better. Because brake pads are significantly better than brake shoes, the government mandates front brake pads.
Most, if not all cars made in the last 10 years use front and rear brake pads. The parking brake only engages the rear brakes, so manufacturers tried to incorporate a parking brake mechanism inside the rear calipers. This system never worked well so they abandoned that system. We now see a hybrid shoes/pads in the rear. When you press the brakes, the rear brake calipers work exactly like the front brake calipers. When you engage the parking brakes, you actuate a small set of shoes which are located inside the hub of the rear brake rotors.
You can see the parking brake shoes since that
The brake rotor has been removed.
The brake pads rub on the outside surfaces of the brake rotor. Over time the brake rotor thins due to this friction. There is a federal minimum thickness listed for all brake rotors. This minimum thickness is different for each car model. When brake pads are replaced the brake rotor needs to be flat. This can be accomplished by resurfacing the brake rotor or by replacing the brake rotor. When re resurface the brake rotor we use a brake lathe which thins the brake rotor and leaves a smooth finish. Ten or more years ago, the brake rotors were thick enough such that we could resurface the rotors two or three times before they needed to be replaced.
I’d say, for the last 10 years, new brake rotors are rarely resurfaced since their thickness are slightly above the federal minimum. The good news is that new brake rotors are significantly less expensive than they were 10 years ago. In many cases the new brake rotors cost less than the cost of resurfacing the brake rotors. Many times I have replaced the brake rotors, not because they were too thin, but because the new brake rotors were cheaper, or the same cost, as resurfacing them.
If the brake rotor ‘warps’ you will get a pulsation in the brake pedal. To eliminate the pulsation you either have the brake rotor resurfaced, or you have the rotors replaced. This is felt in the brake pedal NOT in the steering wheel. On a side note, if the steering wheel ‘pulsates’ it tends to indicate bad tires or an out-of-alignment problem.
This is a typical brake rotor
Brake Master Cylinder:
When You press the brake pedal you force brake fluid from the brake master cylinder to the four wheels.
For safety purposes, the brake master cylinder is actually two master cylinders in one unit. One part pushes brake fluid to the front brakes, and the other part pushes brake fluid to the rear brakes. For example, if the brake line to the rear brakes leaks, you loose all rear braking, but you still have front brakes, and visa versa.
Normally when you stop the brake pedal goes down to a point and stops moving. Even with more pushing, the pedal does not go any close to the floor. That is normal and good. If you have a situation when you are at a stop and the brakes pedal slowly goes to the floor, you probably have a bad master cylinder or a brake fluid leak.
Without these valves, when you would begin to stop the front of the car would tend to “Dip.” With these valves, the rear brakes engage a fraction of a second before the front brakes preventing the front end from dipping. I have never had to replace one of these because they were bad. I have replaced them because I could not remove the brake lines from them without doing damage.
Pictorial -Notice the two chambers A typical brake master cylinder
Brake Hoses and Brake Lines:
Brake lines are steel, and brake hoses are rubber. The steel lines, after many years sometimes leak due to rust. The rubber in the brake hoses rarely go bad, but when they do, the rubber cracks and break. If either the hose or line breaks it becomes obvious because the brake light comes on, your brake pedal goes to the floor, and if you wait long enough, the car will not stop as quickly normal. This diminished braking is very obvious.
Brake hoses Brake line
If you have any questions about your brakes call me at 773 935 2400