Of all the systems that make up your car, the brake system might just be the most critical to your safety. In the early days of automobliles, it was also one of the simplest. Over the years as improvements have been made, the system that has evolved isn't so simple anymore, but it's also much more reliable and safer.

Your brakes work as hard or harder than any other part of the car. They are responsible for bringing a couple tons of rolling metal to stop hundreds of times during your average drive, your brakes do this by converting the kinetic (movement) energy to thermal (heat) energy. All of this heat is generated between the friction surfaces of your brake pads and your rotors.

Here are the most common things that can trouble your brake system and how to diagnose those simple brake problems:

Air in the system
This is usually caused by air getting into the brake fluid area, usually from the master cylinder. As the brake pads wear, the caliper pistons ride farther out of the caliper, allowing more fluid to remain in the calipers. Over time this can add up to almost as much fluid as there is in the master cylinder reservoir. If neglected, this will allow the master cylinder to pump some air into the brake lines. Air is very compressable, whereas brake fluid is not, as long as there is a solid stream of brake fluid between the master cylinder piston and the caliper piston, the brake pedal will be nice and firm. If there is air in the system, the pedal will feel spongy and will go down almost all the way to the floor, maybe all the way, depending on how much air is in the system. The standard way of dealing with air in the brake system is to perform an operation called "bleeding the brakes".

Hard brake pedal
Can be caused by bad power booster, (or loss of vacuum to the booster) seized caliper pistons, seized caliper slides, pinched brake lines, and (rarely) problems with the pedal linkage under the dash. The probable best fix is rebuilt calipers,and new pads.
Brake fade: Two phenomena contribute to brake fade, one is the fact that the coefficient of friction of most substances gets lower at high temperatures, and that most liquids will boil at some temperature, and that gases compress, while liquids do not. When you use the brakes to decelerate 3,000 or 4,000 or 7-8-15,000 lbs of vehicle, they get hot. Very hot. Under normal circumstances this would be no big deal, the heat that builts up in the pads, rotors, and calipers will slowly radiate back to the air flowing over them as the car continues down the road. But you aren't going down the road, you are back on the brakes, doing more decelerating for the next switchback. Instead of cooling off, your brakes are getting hotter. And hotter, and hotter. . .As the pads and rotors get hotter the friction material of the pads starts to separate. The binding agent starts to boil off from the surface of the pad, plating out on the rotor as a dark, paintlike film...coefficient of friction approaches zero, pedal gets hard, but no braking action. It usually takes new pads at a minimum to recover from this extreme overheating.

Brake squeal
This is a high pitched squealing noise, often heard when you are going slow and are not applying the brakes. If it goes away as you apply the brakes, it could be coming from the brake wear sensors. (Also called 'squealers' by mechanics.) They are small bits of spring steel that are attached to the brake pads in such a way that when the pads are about 75% worn out, the sensors start to rub on the rotors, making the noise. GM invented them, and they are one of the best ideas anyone has ever had in the automotive industry. The sound is so scary that you usually go to a mechanic before any major damage is done to your rotors, and before your braking power is compromised, saving you money and maybe your life.

Grinding noise
Although this is one of the nastiest sounds you will ever hear, it often is the easiest to repair. The first thing we must do is learn what is making the noise. We figure out which wheel it is, then, after safely raising and supporting the car, take off the wheel & tire.  We then rotate the brake rotor back and forth until we can identify the source of the noise. Sometimes it is just a small stone, trapped between the brake rotor and the air deflector. The faces of the rotor should be smooth and clean. If we find large scaly rusted places on the friction surfaces of the rotor we will replace them. Most of the time new ones cost less than you would guess. If your pads are worn out (less than 3/16 of an inch of friction material left) and we catch it in time, all you will need is new brake pads. If the surface of the rotor is damaged, they will have to resurface or replace it.
Brake pedal pulsation: There are a lot of things that can cause this, from out-of-adjustment wheel bearings to rotors that are bent, brake drums that are out-of-round, rusty spots on the rotors that have a different surface smoothness. To determine whether the pulsation is coming from the front or the rear wheels, check to see if you can feel the pulsation in the steering wheel when the pedal is pulsating. If you can, the problem is coming from the front wheels. 

Brake pull
Mostly this one comes from either a caliper piston seized or caliper slides seized. This one is dangerous! If your car tries to turn when you apply the brakes you could veer into oncoming traffic. What often happens with this one is this: the caliper piston on one side starts to seize, the other one now applies first, car veers away from bad part. Driver learns to compensate by steering opposite to the pull every time he brakes. A panic situation comes along, driver nails the brakes, steers away from the expected pull, but because the piston was only partially seized, it works just fine when the brakes are applied with vigor. There is no pull this time. It is easy to lose control of your car in situations like this, if your car pulls to one side or the other when you brake, get it fixed before you hurt somebody. Replace calipers and pads and service the caliper slides.
Brake grabbing: When you just barely touch the brake pedal and one or more wheels locks up and skids. This one most commonly comes from contaminated friction material on one or more brakes. Usually we can just clean the pads and polish the rotors to fix this.

Pedal goes to the floor
This has to be the scariest of them all. If you're lucky, a quick pump on the pedal will get you some braking action. On most newer cars, there will be some braking just before the pedal reaches the floor. Stop driving and check your fluid level. It might just need to be topped up to temporarily get you some braking action to get you home. Regardless of what you find, you need to find out what caused it as soon as possible and fix it before you drive any further.