Master Cylinders are used to increase safety. It separates the braking system into two circuits and also supplies necessary pressure to the brake lines when braking. The idea behind this is to allow for the vehicle to stop provided there is a leak in one of the circuits. However, stopping power would definitely be decreased.
Always be sure that the master cylinder if full of brake fluid. Check your brake fluid when you check your other vehicles fluids and be sure that there is no leaks in the brake lines connecting to the master cylinder. If you find a leak, address the issue right away.