I was driving home and when i hit the brakes they were hard and barely stopped the truck. It is a 96 F-350 4door 4wheel drive. the brake light did come on.I change the brake booster and master cylinder and did not fix it.thanks in advance for any help you cangive me.

 

My guess would be that your vacuum pump has failed. Check that the vacuum hose to the booster is good first. Now pull the hose off of the booster and with the engine at idle, put the palm of your hand on to the end of the hose. It should feel like your hand is being sucked into the hose. If not then the vacuum pump is probably bad.
If you can get a vacuum gauge on it you should get around 20in of vacuum at idle.

 

Your right, it's the vaccum pump

 

Vacuum pump failure.