Q: Brake pedal loses pressure

From the description provided, it appears that the symptoms are due to the leak in the brake system that you found in the rear passenger wheel. In order for your power brakes to work correctly, the engine needs to be running. This allows it to supply vacuum to the brake booster, which in turn allows the driver to easily depress the pedal. When the pedal falls to the floor, it is because the brake fluid inside is escaping the system through a leak. This and any repair regarding brakes is a big safety concern and one that needs to be addressed immediately. Enlisting the help of a certified mechanic will allow you to pinpoint the brake fluid leak so that it can be repaired, bled, and rechecked to get your Mustang safely back on the road.