Classic signs of a brake booster failure include a brake pedal that becomes much harder to press than normal, increased stopping distance, and the car taking longer to slow down, even when you push the pedal firmly. You may also notice the engine dropping RPM, stumbling, or stalling when you apply the brakes because a leaking booster or its hose causes a large vacuum leak that affects the engine. Sometimes, there is a hissing noise from under the dash or at the firewall when the brake pedal is pressed.