Calm down mate.
If you can hear air when pushing the brake pedal you can't be certain it is not the seal failing on the input shaft. Common enough and something I have personally experienced. Certainly not something to simply discount at any rate like you suggest.
No sweat mate I wasn’t saying any seals were failing or discounting anything
I clearly stated you can get atmospheric air noise with just pedal movement, similar to a hand pump motion producing squish noises. I certainly wasn:t saying it was air loss from booster diaphragm fail as that would be obvious if it happened to anyone.
If you read the 1st OP post he clearly states that ....
The car stops alright in terms of brake performance but I really have to go beyond half way on the brake pedal before anything starts happpening.
With the car turned off the pedal pumps up fine and goes firm, but with engine running it is very spongey. I get a bit of air noise when depressing brake pedal with engine running. I also noticed my idle can go a little rough depending on my braking so Im thinking my booster is on the way out.
So according to OP brake performance is good, spongy pedal and a bit of air/rough idle when braking, car is old VN so idle probably not perfect for age in the tune, autos, especially when drawing vacuum for brakes or any other part needing vacuum, once driving you wouldn:t notice anything.
He did booster test and ok and still working properly for good brakes.... so it:s not a booster fail....... that leaves the spongy pedal feel, well proper bleeding and different master cylinder conversion change points to be his issue which can produce a spongy feel even when the brakes stop properly and have no major leaks....can also be the booster vacuum check valve poorly working.
So lets not simply discount those items at any rate which you haven:t suggested