I have a Holden 1978 Kingswood with 244,000 kilometres on the clock, 3 spd auto. The brake pedal is solid with the engine off but will sink MOST of the way to the floor when you push it with the engine on, only towards the end of its travel does it become solid. I'm confused because when you turn the car off, you will still be able to pump it a few times and the brakes will feel loose before they go back to being solid. The pedal also doesn't retract the whole way when you take your foot off with the engine on (but does retract all the way with the engine off). The booster check valve is working, I tested it by blowing into its pipe and it didn't allow air to go in, only out.
Please watch the whole video and let me know your thoughts.
You need a new brake booster
