Nothing scary about it mate
it:s just atmospheric air moving from pedal action, not internal pressured air in booster being lost that your implying and after changing many boosters and master cylinders someone like yourself should know that even if yours was just a straight swap of parts used for different options for disk or drum and different engine sizes.
Boosters are just pumps and storage tanks to aid pedal action transfer of force to the master cylinder through the pushrod, so they don:t operate or interfere with the line fluid pressures for brake performance unless someone fiddles with the pushrod length in conversions or pushrod is too long or short for different master cylinder swaps.
Firstly, disc/drum rear came with both 15/16 and 1" bore masters....No difference with them....Go look at VN to VR....Factory disc rears, and 15/16 masters...1" bore was used on 6 banger cop cars around this era too.....Rellie has a VP ex-cop car with single diaphram booster and 1" bore master....Even early commodores with disc rears had 15/16 masters...The 1" bore masters are the harder ones to find...And yes they were fitted to disc reared cars, rather than drum rears....
Booster..The V8's had the double diaphram one...Not sure if the 6 ever came out from the factory with it....And my swapping had zippo to do with engine changes....
If you hear air escaping when you depress the brake pedal, with the engine running, the booster is stuffed....So giving advise that it is normal, is scary advise....