Truck Forums - Brake problems

First time here so bear with me. To start with I have an 1981 GM Sierra Classic 1500. When pushing the brake peddle down you can hear air coming from the push rod / piston that travels threw the firewall, and its pretty load. The peddle doesn't seem to go down any further or much further than it has before ( a couple inches from start to stop ) and dose seem to stop the truck pretty well but if you pump the brakes the peddle gets hard very quickly and doesn't seem to want to stop the truck without really standing on the brakes. When you release the brake for a few minutes it will press as it should as I described earlier. The whole time the brake is held you can hear the air ( loud ) is leaking. There doesn't seem to be any other problems no leaking fluids, all pads are in good shape, master cylinder seems to be fine and very seldom have to add any fluid. The push rod from the brake peddle is not leaking fluid ( which with this truck not sure if it would anyway ). And Im guessing. here but the donut looking metal object that the master cylinder is mounted to and it is mounted to the firwall would be some sort of booster ? The book I use a lot " Haynes Repair Manual " has a couple of shots of it in some picture but no name for it is listed. Can someone tell me what could be the problem and what I need to do to fix it. Could it be the rubber boot around the brake push rod ? From what Ive looked at thats about all I can come up with. You can hear a little air from under the hood while but its hard to tell where that is coming from. But the realy loud noise only happens while the brake peddle is pushed. Thanks for any help I can get with this problem.