I have a 88 f350 dully with a 460 while pulling my 29 foot camper today I pressed the brake pedal they were fine the first time the next time i pressed the pedal it is hard it will not move but maybe 1/2 inch i used the trailer brakes to get me to work but i have to fix it before going camping this afternoon. I can here a hissing sound i was wandering if it may be a vaccum problem.

 

Yes, you have a bad brake booster. The hissing noise is the giveaway..... Welcome to FTE!
Most are easy to change, but look for a hose that has come off first.