The circular fitting in the brake booster entrance hole is a check valve. The procedure goes as follows. Hose for the vacuum comes from the intake with vacuum tree, the brake booster has a rubber diaphram that is pulled foward from the vacuum, when you put your foot on the pedal and press the added vacuum pulling on the diaphram towards the front of the car makes is easier for you to press the pedal down, the check valve prevents all the vacuum from pushing back into the intake. A hissing sound comes from a vacuum hose OR a diaphram leak. I have seen the diaphram go bad on boosters before (not a mustang though) and I have seen the check valve go bad as well. To test the check valve, pull the hose off and pull it from the booster. It should be in there tight. You should be able to suck air from the front using your mouth, but not get anything to pass through when blowing. Vice-versa from the rear of the valve. You can also usually here a bad booster better from inside the car under the dash, as the leak is coming through the booster. Other than these items, somewhere else you could have a hose leaking and when the system is under the gun from the booster being used, it manifest.
