I seem to remember something somewhere about the push-rod or master cylinder piston cup.......the more I think about it the more it seems to cause a brain spark, a bit like a bug zapper not quite zappin that bug. Eventually though, it may come to me, as I can re-call issues with brake parts on HJ's thru to HX being somewhat interchangable, but also needing to be very careful.SO......
Give Jim at Sydney brake and clutch exchange a call on 02 9663 2556.....he may remember better than I can, as he is still hands-on and 50 years(I think) owner of the shop. I will also talk to a mate who woked on H series Holdens for quite some time and see if we can help before you tear all your hair out or develop a drinking problem
If Canberra is closer, try Commonwealth Motors spares dept, as there may still be a bloke there with excellent knowledge about them too. oldholden.com in the holdepeadia might help too, if not contributors on there might know too