Auto safety questions: how power brakes work

The advent of power brakes was a milestone in automotive development. A guide to funcionality, repair, parts.

If you’ve ever driven a vintage car from the early 1950’s or before, you realize what an advance the development of power brakes was. If you were a member of the driving public during the changeover, you probably recall the adjustments that were necessary in driving techniques. The early systems were somewhat touchy and led to many undesired screeching stops but today’s braking systems provide smooth reliable and safe stopping power. And with the advent of anti-skid braking systems, automobile manufacturers have taken another big step in automotive safety.

The basics of the automotive braking system are the same regardless of whether you are considering power or non-power, disc or drum so let’s consider that first. When you push on the brake pedal, you are forcing a plunger into a cylinder (called the master cylinder) filled with brake fluid. Brake fluid is an oily liquid that has features of corrosion and temperature resistance that make it especially suitable for brake systems. As the plunger goes into the cylinder, fluid is forced out into metal tubes called brake lines. These lines run from the master cylinder which is usually mounted just behind the brake pedal to each of the wheels. At the wheel, the line connects to another cylinder (called the slave cylinder). As the fluid enters the slave cylinder, its plunger is forced outward and presses the brake pad or shoe against the spinning wheel. The friction between the shoe and wheel cause the wheel to slow down. By adjusting the relative sizes of the master and slave cylinder pistons, you can magnify somewhat the force applied to the master cylinder. This effect is however limited by physical constraints such as where the cylinders have to be placed.

The idea behind power brakes was to find a way to magnify even more the pressure applied to the master cylinder so that even relatively weak drivers could apply the maximum force and stop as quickly as possible. They decided to use a variation of the same principle that was already in use in the master and slave cylinder arrangement. Suppose I have a hollow sphere and stretched across the middle of the inside of the sphere is a flexible wall called a diaphragm. If I start to remove air from one side of the sphere and allow air to enter the other side, the diaphragm will bend toward the side with less air (lower pressure). Now if I attach a rod to the center of the diaphragm on the low pressure side and then attach that rod to my master cylinder plunger, the rod will push on the plunger and apply the brakes. How hard it pushes depends on how big the diaphragm is and how much air is removed. So the first step in developing power brakes was to attach a flattened sphere with a large diaphragm inside to the master cylinder.

The next question was how to remove air from the sphere. As it turns out, a running engine is constantly sucking in air to mix with gasoline so it was an easy task to attach a small hose from the engine to the sphere and let the engine take air from it. Of course we only wanted it to be connected when we applied the brakes so a valve was needed. Now we attach our brake pedal to the other side of the sphere so that when the pedal is pushed it pushes on the other side of the diaphragm and our system is complete.

So when you step on your power brake pedal, two things happen. The brake pedal pushes on the high pressure side of the diaphragm and therefore on the rod attached to the low pressure side and then on to the master cylinder. This is important because if the power booster unit (this is the official name of the sphere) fails, we still have manual brakes. At the same time, the movement of the brake pedal opens the valve that allows the engine to remove air from the low pressure side and the diaphragm moves and applies even more pressure to the master cylinder and therefore the brakes. So the pressure of our foot is magnified and the brakes are applied much harder for extra stopping power. There is much engineering work involved in finding just the right size diaphragm, making sure the valve opens properly (not too fast and not too slow) and matching all the other components for smooth reliable operation but the basics remain the same.

Now another problem arose. Cars stop fastest when their wheels do not skid. The shortest stopping distance is achieved when the brakes are applied such that the wheels are almost but not quite skidding. Now that power assist gave anyone the ability to apply maximum force to the brakes, skids became even more likely. So the next step was anti-skid braking systems. When computers became a viable component of automobile systems, sensors were mounted on each wheel. These sensors monitored the movement of the wheels and when the computer detected that a wheel had stopped spinning but the car was still moving, it released the brake on that wheel until the skid stopped and then reapplied the brake. This happens much faster than a driver could react and means that the car will stop in the shortest possible distance.
Brake systems today are typically broken into two separate systems so that if one fails the other can still stop the car. Combined with anti-skid brake systems, this provides a level of safety and reliability that was unheard of even 10 years ago.