So, I bled the brakes using Motive Power Bleeder, starting with the RH rear corner, ending with the LH front. Some bubbles, but not many extruded through the clear plastic bleed hose. It's no different than before, still sinking. It's perhaps somewhat worse than when we started. The new master cylinder comes with a new O-ring between MC and booster, which was installed. Going for a test drive through the quiet neighborhood now, will advise results.

I'm thinking booster diaphram.

77 300D