The power brake booster on your Ford vehicle uses vacuum created by the engine to apply extra power to the brake pedal, which increases braking ability and decreases pedal feel. If the booster develops a leak or starts to have issues, you'll have increased pedal feel, and although you won't lose your brakes entirely, it will become more difficult to stop as the leak gets worse. In this case, the project vehicle is a 1997 Ford F-150 truck, but the process is similar for other vehicles.
Crawl underneath the dashboard and follow the brake pedal up under the dash until it connects with the brake booster. Disconnect the clip that connects the booster to the pedal, and then slide the booster linkage off of the pedal assembly.
Peel back the firewall padding, and then use the 3/8-inch ratchet, extension and sockets to unbolt the brake booster from the firewall. You may need a 3/8-inch universal joint to access some of the bolts.
Pop the bonnet and unbolt the master cylinder from the brake booster, using an open-end wrench. Pull off the vacuum line from the booster, then pull the master cylinder away from the booster, and let it hang off the brake lines.
Pull the brake booster off of the firewall and place it to the side.
Slide the replacement booster into the firewall, then crawl under the dash and bolt it in place, using the 3/8-inch ratchet and socket.
Reconnect the pedal linkage to the booster, using the factory clips.
Go under the hood and slide the master cylinder onto the mounts on the booster and bolt it in place, using an open-end wrench. Plug the vacuum line back in place with a firm push.