The pressure provided by the Pontiac brake master cylinder is the stopping force of your braking system. If this component fails for any reason, your ability to stop your forward momentum will be compromised. The Pontiac brake master cylinder is activated when the brake pedal is pushed. To assist in applying pressure to the brake master cylinder from the brake pedal is the vacuum drum (called a booster) on which the master cylinder is mounted to. This vacuum assistance is only available when the Pontiac engine is running. When the vehicle is turned off, the depressing of the brake pedal is significantly stiffer because there is no assistance from the vacuum drum without the engine running. The brake master cylinder creates pressure in your braking system by moving a piston that forces hydraulic fluid into the brake lines and hoses. The pressure is maintained in the lines because there are seals on the piston that prevent it from being released. It is these polymer seals that are the only component that can fail on your brake master cylinder. When these seals rupture, the hydraulic fluid is permitted to seep past them and the pressure being built up by the pistons movement is then limited. This reduces the pressure in the brake lines and hoses so the stopping potential of your Pontiac is severely impaired. Symptoms of this occurring in your brake master cylinder is a soft brake pedal where pressure cannot be maintained, and it will travel all the way to the floor when pressure is being applied. Unlike other braking components when the polymer seal fails and a puddle is formed, the piston is inside the cylinder and no exterior leak can be seen under your Pontiac.