This is a cross-section illustration...
This is a cross-section illustration of a vacuum power booster at rest. Vacuum from the intake manifold is closed off from the booster chamber when the brake pedal is at rest.
How Power Brakes Work
What exactly are power brakes and how do they work? All Mustangs produced since March 1964 have been equipped with two basic kinds of power brakes. Most common is vacuum-assisted. Vacuum assist helps us apply the brakes, reducing pedal effort.
Vacuum-assisted (or boosted) power brakes were used in Mustangs until 1996 when the huge 4.6L SOHC and DOHC V-8 engines filled the engine compartment to capacity, making it impossible to fit a vacuum booster between the engine and inner fender. Thus, Ford went to hydroboosted power brakes, which consume less space. Hydroboost gets its pressure from the power steering pump.
Vacuum-assisted power brake boosters get their vacuum from the engines induction system (intake manifold). When were running a really radical engine, intake manifold vacuum suffers, making it necessary to use an electric or engine-driven vacuum pump to help the power booster.
Hydroboost is nothing new. Its been used in Lincolns, Cadillacs, and other types of luxury cars for ages. It works on the same basic principle as power steering. We use hydraulic pressure from the power steering pump to help apply the brake pedal. This is the same basic principle as the Bendix power-assisted steering common to 1965-70 Mustangs. Move the steering wheel and you tell a control valve to send hydraulic pressure to one side of the steering ram or the other. This helps you steer. Same idea with hydroboosted power brakes. Touch the brake pedal and you tell the hydroboost to apply pedal pressure.
Did you know you can install hydroboost power brakes in your classic Mustang? Just go to the Lincoln parts shelf for answers. However, hydroboost power brakes can be more high maintenance than vacuum assist.
 When we step on the brake...  When we step on the brake pedal, the booster's diaphragm squeezes the air in the front side of the booster. This pressure unseats the check valve that controls intake manifold vacuum. When the check valve opens, manifold vacuum sucks the air out of the booster. This "suction," called negative pressure, pulls the diaphragm forward, which applies forward pressure on the rod and master cylinder piston. The power booster helps us apply the brakes. |  With our foot on the brake...  With our foot on the brake pedal, intake manifold vacuum continues to hold the diaphragm against the rod to the master cylinder. If we're just sitting there with a foot on the brake pedal, the power booster really isn't working. It works only when we initially apply the brakes. When the diaphragm moves forward and applies the brakes, the vacuum check valve closes and the vacuum fades. |  This is a cutaway of a typical...  This is a cutaway of a typical Mustang power brake booster (circa 1965-1966). As you can see, the power booster "helps" us apply the brakes, reducing pedal effort. |