No pedal on brakes, when I push brake pedal to the floor I hear a hissing noise in the cab of the truck (96 Dodge Ram 1500)

Had the same problem. I just replaced the brake booster unit, had a vaccuum leak out of where the push rod goes in. Solved.

The hissing noise is not important.The pedal to the floor is.They do not have any thing to do with one another.The pedal to floor can be,a leak some where,so check all four wheels for fluid leakage.Then check the master cylinder for leakage,and if the reservoir to see if it is empty,or almost empty.If there is no fluid lose at all,and the reservoir is at the right level,or close,then remove the rear brake drums,and inspect the shoes,to see if they are worn out.If they are o k,on both rear wheels,then put the drums back on.Adjust the rear brake shoes to where the brake drums drag on the shoes ,and just a little more.Do not adjust to tight.You need to be able to turn the drums without a whole lot of effort.O k.now if the brake pedal does not come back up,then change the master cylinder out with a new one,or a new remanufacture.This should fix ya,let us know if it does help you,please.Thank you.