Vacuum Brakes vs. Vacuum-Assisted Brakes

• There are two types of "vacuum brake" system: the vacuum-assisted system used in automobiles and the true vacuum brakes once used in trains. A true vacuum brake system is functionally almost identical to the air brake system used in large trucks. In an air brake system, compressed air pushes on a diaphragm attached to a rod that pushes or pulls on the brake shoes. A spring located behind the diaphragm actually supplies the braking pressure. As long as there's compressed air pushing on the spring, the brakes stay released and the truck is free to roll. A "true" vacuum" brake system does the same thing, but uses vacuum to suck on the diaphragm from the back instead of compressed air to push on it from the front.
  
  

Hydraulic Brake Basics

• An hydraulic brake system is basically just a means of transferring motion from one place to another; namely, the motion of your foot to the brake calipers. When you press the brake pedal, the pedal pulls or pushes on a rod attached to a piston in the brake master cylinder. This piston pushes fluid out of the cylinder and through the anti-lock brake modulator valve. As long as the ABS valves are open, fluid continues to travel from the master cylinder to the brake caliper pistons, which squeeze down on the brake pads. The master cylinder is typically longer and thinner than the caliper slave pistons, which means that the slave pistons will move less distance, but with more force.
  
  

Vacuum Assist

• The vacuum booster on your hydraulic brake system is similar in principle to the vacuum system used in trains, varying mainly in location and power. Whereas the train system uses one or two vacuum chambers and diaphragms per axle, your car uses a single vacuum diaphragm that acts only on the master cylinder piston. This diaphragm mechanism sits in-between the brake pedal actuator rod and the master cylinder. Under normal conditions, a hose connected to the engine's intake manifold sucks all the air out of both sides of the diaphragm. When you hit the brakes, an air valve on the back of the booster opens. This difference in air pressure shoves the diaphragm forward and adds some force to the master cylinder's piston movement.
  
  

Vacuum Assist vs. Manual Brakes

• Vacuum-assisted brake systems generally use much shorter master cylinders than standard braking systems, which gives them less mechanical advantage over the caliper pistons than non-assisted brakes. This means two primary things to you as a driver: Brake pedal travel on a vacuum-assisted system is far shorter than on a manual system, and the brake pedal on an assisted system may get rock-hard if you lose vacuum for any reason. Longer pedal travel is a bit inconvenient for street use, but it does allow for more precise brake pressure control and feedback. Additionally, engines using large, rumpety cams may not produce enough vacuum at idle to activate the vacuum booster. These two factors are the primary reasons why manual systems often find a home in dedicated race cars, where braking precision and vacuum both come at a premium.