I am looking for guidance on the brake system for my 1997 GMC Yukon. The brakes work well enough, but they work far better if I pump the pedal. For instance, if I approach a red light and push the brake just once, it will stop, but the pedal is pretty soft. If I push the pedal, quickly let off and then reapply, the brake is firmer. If I do that a second time, it gets very firm. Also, when I apply the pedal, there is a noticeable hissing/air leaking sound. When I bought it in August, I machined the front rotors and put on new pads. I then bled the brakes at all four wheels and replaced the brake fluid. That didn't change much, if anything. I also adjusted the rear drums and they seem good to go. So I spent $28 and replaced what appeared to be the original master cylinder with a new Dorman one. I bench bled the new MC and then re-bled the truck at each of the four wheels. Unfortunately, the pedal feel hasn't changed. At this point, I'm thinking it's the booster. My question, though, before I throw a fairly pricey part at is this: Would a failing booster produce the hiss and soft pedal (that can improve with pumping)? If it is a booster, would it make any sense to try and find a salvage yard special? Based on the age of anything sitting in a junkyard, though, that may not be wise. Thanks.