Double pump brakes

The car is a 71 duster with a hodgepodge of brake parts on it, scarebird conversion up front, 68 dart 8 3/4 on the rear (with the associated brakes) and a newer master cylinder (I believe it's a double reservoir or whatever it's called)

When I'm driving it, often times the first pump of the brakes goes to the floor, the second is nice and solid

I bench bled the master when I swapped it out (a few years ago)

What should be the first thing to do...bleed all 4 brakes ?