I got a 2000 buick century custom when I got it it had really good breaks but now they are not hard as they used to be what should I do?

Completely change the brake fluid, using Valvoline SynPower brake fluid. This fluid is made for ABS brake systems and has higher wet and dry boiling points than most commonly available brake fluids. Changing the brake fluid will eliminate air, moisture, and crud in the brake system. Additionally, it will replenish the lubricity and corrosion protection properties of the fluid. You want to do this every 2 years. This is one of the keys to having problem-free brakes.