<Snip> Whenever I needed to brake, the pedal would slowly push all the way to the floor.  I could stop, but it sure made me nervous.
<Snip>

I had this problem a while back and, in my case, changing the master cylinder didn't correct it.  I sure THOUGHT it would, though,
as that is a classic symptom of fluid leaking past the seals.

I chased down vaccum leaks (due to a different problem), replacing all small hoses, replacing the factory crimp on clamps with hose
clamps on the 4 short braided hose connections at the intake air distributor, and ... the two things that made the most improvement,
re-sealed where the oil dip stick tube enters the engine case and where the plastic part of the oil fill tube joins the metal part.

Strangely ... haven't had the sinking brake pedal since .... about 8,000 miles worth.

Maybe the power assist was having problems due to the vaccum leaks.

Mike
'84 GL "Mongo"