1999 burb brake system

I have a question about the brakes on my 1999 Suburban 4x4. The brakes seem spongie. They go about half way to the floor before they start to grab. Then they will not stop the truck when they should or at least with in the time and distance that I think they should. What is my problem and how hard is it to fix?

How manny miles do you have on the car?
Have the brakes ever been serviced?
Are there any grinding noises ?
Did this problem just start,or has it benn going on for a while?
Look for fluid around the inside of the wheels. You may have a leaking rear brake wheel cylinder. Open the hood, and check the master cylinder reseviour.