The pedal only falls away when a light pressure is applied to it. This because a slight movement of the pedal takes up the slack between the pedal pushrod and the MC piston rod. A small movement of the MC piston opens up the inlet behind the piston and closes the inlet for any required make up fluid in front of the piston, I’ll ignore the secondary piston other than add that it starts to move after the primary piston.
By this time the diaphragm of the booster has started to move as the vacuum valve has closed and atmospheric pressure has started to push the piston. Even if the booster is defective how come the pedal falls away so much? That is the explanation I am seeking.

In the meantime I am out tomorrow to find replacements for my rear Hankooks, they are down to the wear bars on the inside and 1/32” away on the outside. The fronts are evenly worn with a lot of tread left. The TPMS has been disconnected and went dead last year after 16 years and 40,000 miles. The fronts were aligned 10 years ago but they could not do the rears, evidently they did not need it anyway.