Hear air when I step on brakes

Have a 87 full size blazer, 305ci, 4x4. Last week my brakes started going to the floor. No leaks and I wasn't losing brake fluid. Someone told me to replace the master cylinder, I did that, pedal really isn't that hard. Everytime I step on the brakes, I hear a burst of air under the dashboard where the brake pedal is. It sounds like someone is letting out a big exhale everytime I hit the brakes. Can't remember if this sound was there before or not. Should I be hearing a sound like that?
Sounds like you need a new brake booster.

Steve
It very well could be a bad booster but if there is excessive brake pedal travel the sound that he’s hearing maybe the vacuum normally escaping from the booster during a pedal application.
With the brake pedal in the released position, vacuum is applied to both sides of a diaphragm. As you apply the brake pedal, an air inlet valve opens allowing vacuum to escape and atmospheric pressure to enter the one side of the booster. With the low pressure (vacuum) on the other side of the diaphragm the atmospheric pressure pushes the diaphragm towards the master cylinder; increasing the pressure applied to the master cylinder’s pistons.
I would go back and check all the basics first as in rebleeding the system making sure that all the air is gone and then checking the wheels for adjustment and proper function. No sticking or binding caliper guides or pins anything that would cause extra friction material travel.