1978 Chevrolet Silverado

I stopped at a red light today and my brake pedal went to the floor. I can slow the truck down abit but lost alot of the brakes. I checked the break fluid and the back silinder was low. What do I need to do now to repair the breakes? When I step on the brakes I can hear some air sound.
April 8, 2013.

You have blown a brake line. Because of the year, chances are one of the steel lines has rusted and leaked. Check all brake lines and lines for leaks. If they are all good, and I doubt they are, check rear wheel cylinders and front calipers for problems.

Also, refill the master cylinder and see if after pumping the brakes it goes low again. It should and you should see brake fluid in the area of the leak.

Let me know what you find.