'75 Duster intermittent brake problem

Master cylinder is going.
When it's cold out the rubber cups are hard and they don't flare out and seal.

There is a diagnostic I use for this.
With a very light pressure slowly push the pedal down and it will probably go all the way to the floor, and sitting in traffic if you let up on the pressure a little it sometimes will also go to the floor. (Especially cold)