Brake pedal vibration generally caused by aged brake drums and very few times due to other reasons covered below. When the brake drums (aka discs), the braking component whose main job is to hold the brakes gets worn out, the vibration takes place when you stop. When you press the brakes, your car brake pad hold the brake drum slowly that is mounted inside the wheel. Brake drum have certain minimum thickness depending on each car weight and power and when this minimum thickness threshold reaches the first sign usually appears is brake pedal slight vibration while hard braking and occasionally on soft braking. If your car is equipped with an Anti-lock braking system then you should be getting an (ABS) warning that may light up when the car is vibrating. It means that you have to get the brake drums changed. If you do not have ABS, get your brakes checked immediately.
Brake pedal vibration in some cases can also be caused due to brake pad issues; if your brake pads had frequent contact with oil, dirt, or other substances. It can also happen if the pads are damaged or having any manufacturing defects or poor quality duplicate brake pads. Another cause could be the age of brake disc. As with age, discs grab brakes unevenly, without warning cause the pedal to vibrate. It could be because of overtime wear. It is important to always replace both discs on front or back at the same time, even if one is causing harm.
Next comes the wheel alignment issue. If the car is out of alignment, pedal vibration is obvious. Non-alignment results in rapid tire wear and harm to the suspension components. To address this, get your car up to 80-100 kmph on a straight road and leave the steering wheel for a moment. See if the car immediately tries to go to the right or left. If it does, realignment is required. Also check on worn suspension components like ball joints, tie rods, wheel bearings, and others, as in some cases failure of these wheel related component can cause such brake pedal vibrations.
Visual inspection by yourself of brake drum and pads (in some cars) can tell lot of first sign of fatigue and then you or your mechanic can check for advance level doubts like: cracked CV joint boot or a hairline crack on the rotors, low brake oil, master cylinder is misbehaving, brake drum is not fully placed inside the hub, drums have uneven wear that can be fixed by facing them in the lathe machine..
If you need more help, or need specific answer for any question then try the ‘Carnity Forum’ (http://carnity.com/) section or find car businesses near you with the Carnity ‘Business listing’ (http://carnity.com/business_listing) section.