knocking noise under hood

there is a loud knocking noise under the hood when i apply the break. the car is constantly running hot even when i put coolant in it. it's a2001 Chevy Malibu. Any suggestions?

Your knocking noise sound like you could have a brake booster going bad. What do you feel in the brake pedal? do you feel the knock?
 
By "running hot" you mean at temps above 240 degrees F? 

Need more input!!!  How are the brakes? Do they stop the vehicle in a average amount of time?? Does it sound like it is metal to metal?

And im agreeing with Euro  how hot is hot?? And do you always have to add coolant?