As a person who owns a few cars. There is a chance that you will encounter some problems with one car shaking while you brake. Normally it is not a big deal but if not looked at it could be a bigger problem than it needs to be.
Now I am going to go over why your vehicle may experience shaking. I am going to list the most common reasons from most common to least common.
Break Pads and Rotors
This is the most common reason for a vehicle to be shaking. When the brake pedal is depressed, the caliper applies pressure to the brake pad. The harder you press the more pressure is applied.
The caliper pushes the pad into the rotor to stop the vehicles wheels from moving. Over the course of a vehicles lifespan especially colder states snow,dirt and other materials may get in between the pads.
So overt time rotors and pads get thinner making them easier to damage. While braking the friction between the pads and rotors causes immense heat which can cause the rotors to warp. This can also lead to further problems such as slipping brake pads.
To save yourself from these troubles make sure pads and rotors are checked in between every oil change. The more you brake, Or harder the sooner they need to be replaced.
If you don’t get the pads checked out there may be a loud grinding noise coming from the outside of the vehicle. This is a must need new brakes and rotors, If this is left unchecked you may have caliper problems and brake fluid leaking.
Guide pins/ Stuck Caliper
Dry/Stuck guide pins could be the main issue if the steering wheel shakes while coming to a complete stop. Guide pins are essential to the brake assembly as it guides the brake pads into the rotor so they do not wear unevenly.
To prevent uneven wearing every time the brake pads and rotors are done these pins are lubricated and cleaned so that the can move freely and without any dirt or moisture in the boot of the pins.
Pins will only get stuck if not lubricated correctly or if the boots on them got ripped causing dirt and moisture into it causing little or no movement.
Stuck calipers can happen in 3 ways the boot of the caliper got ripped and now the brake fluid leaks out, Dirt or moisture build up inside the ripped boot. The last way is that the pads are so worn that it is sticking out so far that its rubbing into the rotor.
There may be an alignment problem if braking at high speeds such as 50 mph causes shaking. To verify that this may be the problem let the steering wheel go around that speed and see if it shakes at all. Be careful to not crash the car as this is a test not just letting god take the wheel. Only do this for a few seconds to make sure.
If the vehicle does not go to the left or right and goes in a straight line then it may not be an alignment but another concern.
This is a very common and inexpensive way that may cause the vehicle to shake while braking. If there are any unbalanced tires this will cause a massive shake in the front and or rear only at highway speeds. This would be a good time to check out if there are any uneven tire wear and if it needs and alignment.
Also rotating tires is a major factor to this. It is important to get tires rotated to prevent unbalance and prevent uneven tire wear. Techs should check tire wear every oil change and let you know if a rotate is needed.
Bent Wheel Rims
When a balance is done a tech should inspect if a wheel is bent. If the wheel is bent they can balance it if its not terrible. Going over a pothole or a curb may cause a rim to bend which throws off the balance of the tire. If the bend is to much over a distance a new rim will be needed in order to ensure the safety of the driver.
If you ever have a problem where the car and/or the steering wheel shakes. Take it to a shop and verify the problem most of the time it is a balance or Brakes. So in any scenario be ready for worst case as vehicles are not cheap and small problems can become bigger ones the longer you let the problem occur.