Monte Carlo's brake pedal goes all the way to the floor

Usually you end up ruling everything else out, leaving the master cylinder as the suspect. Was any work done recently on the hydraulic system? Have the rear brakes been inspected?

A low pedal is caused by one of three things. There is an external leak and you should see a puddle by one wheel or along the left side of the car. There is a mechanical issue such as rear shoes out-of-adjustment or one lining has rusted off the frame. Adjustment problems gradually cause a lower and lower pedal over a long time. If the pedal was fine one day and goes all the way to the floor the next day, check the rear shoes. The third thing is internal leakage inside the master cylinder. That always starts out affecting half of the brakes. While the pedal will go too far to the floor, the car will still stop with one front brake and the opposite rear one. The red warning light will turn on too.

Air in the fluid is another cause of a low pedal but that doesn't occur on its own unless the fluid level is empty in the master cylinder reservoir.

Even if there's an external leak, GM front-wheel-drive cars have an unusual valve in the master cylinder that blocks the ports leading to the leaking circuit. That prevents further loss of brake fluid, and most people don't even notice it. The pedal will still feel normal too. With internal leakage in the master cylinder, the pedal could go to the floor.