Brake Booster and Master Cylinder —The brake booster and master cylinder is the heart of the brake system. Brake fluid is added to the cylinder and the booster is the vacuum source. When you step on the brake pedal inside the vehicle, vacuum from the brake booster forces the brake fluid inside the master cylinder to each wheel. The booster gets its vacuum from the engine via a rubber hose. The booster is what gives you the “power brakes”. Without the booster you will have to manually force the fluid to each wheel, this can take a lot of foot pressure.