Feeling a wobble in your vehicle's breaks may mean you have a serious problem and should be examined as soon as possible. First you should differentiate between brake wobble and other vibrations that may occur when you apply your brakes. They're not always the same, but the cause is similar: something is worn out, worn unevenly or out of balance. The speed at which you first apply your brakes may give you a clue to the underlying cause.
Worn Rotors and Pads
Brakes live in a world of friction. They wear out. Breaks aren't intended to be used until every millimeter of the break pad is worn to dust, but people do just that. When they do, metal-to-metal contact occurs, essentially turning your front wheel hubs into lathes. The metal contact will wear into your discs at a faster rate than the brake bad. This wear will predictably cause mild to severe brake wobbles. When it happens, you'll need to have your rotors machine-balanced and reinstalled with new brake pads.
Alignment
Other sources of braking-related wobble, are indirect; they're not necessarily due solely to worn pads and rotors, but often happen in conjunction with brake wear. Poor alignment is a common culprit. The front wheels should track parallel to one another, but they can be jarred or vibrated out of alignment. This results in an asymmetric force that results in a harmonic vibration when you brake. It can happen without any rotor and brake wear, but if the alignment vibration continues, it will quickly cause brake wear.
Tires
Worn tires -- especially tires so worn, they have a blister -- will predictably cause mild to severe wobbles. While a severely worn tire will probably be noticeable under acceleration, surprisingly, it's possible for worn tires to only make themselves known under braking. Note, this is an extremely dangerous condition and should be fixed immediately. You are safer driving on your spare.
Suspension and Wheel Components
There's an interrelationship between tires, alignment, brakes and suspension components. Of each of these components, tires wear the most quickly. Tire wear can cause alignment problems. Alignment problems can cause vibration and brake wear, which can cause more vibration. When you get vibration in your wheels, every suspension component will is stressed. Tie rods, hub bearings, shock absorbers, will wear unevenly or otherwise be damaged. Once a vibration starts, you should address the underlying cause right away as it will only cause more problems and greater danger. Be careful you don't fix just the symptom or spend money on misdiagnosed causes.
