Hissing sound from brakes

[Dragstart]

I first noticed a hissing sound when I would have brake pedal depressed at stop light/sign. It has now progressed to being able to hear it without pedal depressed. But, if I depress pedal all the way down, hissing stops.
Of course, it's engine vacuum, but my question is what to suspect? Is it the check valve, (which seems to be OK, removing it and pulling and pushing air through it.), or is it a hole or tear in the booster diaphragm?
Brake function is normal, but can/will the booster finally completely fail?

Or is there a seal around the brake pedal rod where it goes into the booster that is failing?

Thanks!

 

[Uncle Gump]

It's most likely a bad booster diaphragm.

Which means you need a new booster.

Not long ago I had to put a booster in our Edge... same symptoms.