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.
When you have a bad or leaking brake master cylinder the brake pedal will slowly go to the floor as you hold pressure with your foot. You can pump the pedal with your foot a few times to regain temporary pressure. You have a brake fluid leak somewhere or there is an internal problem with the brake master cylinder itself and needs replacement.
A bad brake BOOSTER will usually give you a very hard to push brake pedal, even worse than the old time “manual brakes”. You might have a vacuum leak under the hood leading to the booster or a bad booster which needs replacement.