Here is a peek into a brake vacuum booster with the left facing the front of the vehicle and brake master cylinder. The back (right) is where a pushrod from the brake pedal would enter the system. We’ve shoved a spacer in the booster between the back of the booster and a large rubber diaphragm to simulate what it looks like when your push the brake pedal. As the pedal is pressed a valve opens on the back of the booster, the housing of which is under vacuum. With the valve open on the back of the diaphragm, the engine’s vacuum helps apply pressure to the master cylinder via another pushrod. That’s how “power” assist helps with slowing your 4x4. Release the brake pedal and the valve closes and both sides of the diaphragm receive vacuum and the diaphragm returns to the back of the housing with help from the spring.