A friend of mine have changed the rear wheel cylinders and now the pedal needs two pumps to be ok. I helped him bleed the brakes today and also adjusted the rear brake shoes, but it is still the same. After 30 seconds it needs two pumps again. Made me remember something about there being some problems with some aftermarked rear wheel cylinders?! Some of them missing a spring inside, making the piston slide back to much causing the brake pedal to "floor" at first time depressing it, but with the second try it's ok? There was no springs in the old cylinders. (2cv6 from 1986)

Am I onto something? Or maybe try a different master cylinder?

Yes you're on the right track.
Bendix maybe others are fitted with shit O rings. They roll when you hit the pedal pulling the pistons back when you let go of the pedal. Replace the O rings with these kits and problem gone

Some of the wheel cylenders had dust seals that are too stiff and they add to the pulling the pistons into the cylender

springs like you used to get can be fitted

Sean