You hit the brakes, and the pedal pulsates under your foot. Maybe you even get a little wheel shimmy. Here are a few basic items you should check.
A very common cause is excessive disc runout or a drum that's out of round. Most cars have disc brakes on the fron wheels and drum brakes in the rear. With disc brakes, when you press the pedal two pads squeeze against the disc, causing the car to stop. With drum brakes, two brake shoes inside a drum press out against the drum.
Sometimes, however, the drums become not quite so round, and the discs have excessive side-to-side movement as they turn. Then, as the pads and shoes try to follow the uneven contours of the discs and drums, you get a wobble.
This isn't something you can detect with the naked eye: We're talking about thousandths of an inch here. Nevertheless, a brake disc or drum several thousandths of an inch past the manufacture's recommended specs can cause a pulsating brake pedal. A mechanic can check discs and drums and grind the disc or drum true again.
Another possible cause is badly worn pads or shoes. Brake pads or shoes should have at least 1/16" of friction material. If some of the friction material is completely gone, the discs and drums will be scored. If the scoring isn't too deep, a mechanic can still true the disc or drum.
Never install new brake pads or brake shoes on discs or drums that are scored; have the disc or drums trued first. If they are too badly damaged to be trued within the manufacturer's specs, the offending drum of disc must be replaced.
Grease oil or other fluid on the brake discs or drums can also be the culprit, and this you can tell by looking at the pads or shoes. Pads or shoes that are contaminated with grease or fluid that can't be removed with a brake cleaning fluid will have to be replaced. Use only brake cleaning fluid to clean brake pads and shoes, or the braking surface of a disc of drum. This fluid is formulated to remove contaminants and leaves no residue.