Ford Brake Booster
The function of the Ford brake booster is to assist the driver in compressing the brake fluid so the braking process is easier on the driver. The brake booster is located on the firewall under the hood of your Ford on the diver side of the vehicle. Mounted to it is the brake master cylinder.
All Ford brake boosters are vacuum driven devices. This is why there is not assistance when the car or truck is not running; there is no vacuum being produced by the engine's intake. If the brake pedal becomes harder to push down, this braking component should be the first device that is suspected of failing or not working properly. The feel on the brake pedal will be more than what is felt on an old style manual braking system.
There is a large hose leading to the brake booster. With the Ford engine running, this hose should be pulled and the amount of vacuum being supplied to it tested or checked. If there is not a strong vacuum at this connection, the engine should be checked for a vacuum leak. If you have a strong and steady supply of vacuum then the brake booster should be replaced so your problem can be resolved.
On your Ford, the brake master cylinder will have to be loosened and moved forward to replace the brake booster. If care is taken, the closed loop brake system does not need to be breached. If you must remove the brake master cylinder and disconnect the brake lines, then this closed loop system is opened allowing air to enter. Once the new component is in place the brake master cylinder reattached, the brake system will need to be bled of any air that has entered the system.