How your brakes work

When you press the pedal, you push fluid through brake lines to a caliper at each wheel (or a wheel cylinder, on some rear drum brakes). The caliper squeezes pads against a metal rotor that turns with the wheel, and friction slows the car. The pads wear down by design. Rotors wear too, more slowly. Brake fluid ages, and hoses, lines and seals can leak or seize. A brake job is about keeping all of that in working order, not just replacing pads.

Signs it's time

Squealing. Many pads have a small metal tab that squeals when the pad is worn thin. It's an early warning.
Grinding. A harsh grinding sound often means the pad material is gone and metal is rubbing on the rotor. Stop driving as soon as you safely can; it damages rotors quickly and reduces braking.
Pulsing pedal or shaking wheel. Often a sign of uneven rotor thickness or deposits on the rotors.
Pulling to one side. A caliper may be sticking, or a hose may be restricted.
Soft or sinking pedal. Air in the lines, a fluid leak or a failing master cylinder. This is a safety problem; get it checked before driving further.
Brake or ABS warning light. The red brake light can mean low fluid or a parking brake problem; the amber ABS light means the anti-lock system has found a fault. Both deserve a diagnosis.

What a complete brake job includes

A pad replacement done properly includes more than new pads:

inspecting the rotors and measuring their thickness against the minimum stamped on them;
resurfacing or replacing rotors that are below minimum, warped or scored;
cleaning and lubricating the caliper slides and hardware so the pads move freely;
replacing worn clips and hardware;
checking calipers, hoses and lines for leaks or sticking;
checking the fluid level and condition;
a test drive and, for new pads and rotors, bedding them in.

Brake fluid

Most brake fluid absorbs moisture from the air over time. Water in the fluid lowers its boiling point and can corrode parts inside the system. Your owner's manual lists the maker's recommended interval for changing it. A shop can test the fluid; if it's due, ask for a flush rather than a top-off.

Pads and rotors

Pads come in different compounds. Some are quieter and dustier, some last longer, some handle heat better for towing or mountain driving. Rotors can sometimes be resurfaced, if there's enough thickness left; otherwise they're replaced. Ask which pads the shop will use, why, and whether the rotors will be resurfaced or replaced.

Anti-lock brakes

The anti-lock system uses wheel-speed sensors to keep wheels from locking under hard braking. A failed sensor or damaged wiring is a common cause of an ABS light. When the light is on, the normal brakes usually still work, but the anti-lock function may not. Get it diagnosed.

Questions to ask

What did you measure on the pads and rotors?
Will you resurface or replace the rotors, and why?
Which pads will you use, and why those?
Will you service the caliper slides and replace the hardware?
Is the brake fluid due, based on testing or the maker's interval?
What's the written warranty on parts and labor?