Power Brake Boosters!
Power brake boosters work by enclosing a diaphragm attached to the rod from the pedal linkage inside of a canister, then using vacuum created by the engine (gas powered) or vacuum pump (diesel powered). When the pedal is pressed a valve opens the back side to atmosphere pressure. Since the atmospheric pressure is much higher than the vacuum on the opposite side of the diaphragm, pushing the pedal down becomes much easier. When the pedal is released, this valve closes and the entire enclosure is again a vacuum.
Power brake boosters can become cracked, the check valve break, or the seals wear out. When this happens, the vacuum will not be strong enough to provide sufficient power brake assist and the pedal becomes very stiff.
Consult with an automotive service professional if you feel like your power brake booster needs replacing.
We sell quality power brake boosters from these manufacturers: