Attached is a pic of the combination valve that is in the car.
It works as a T for the front brakes. Other than that it had the warning switch/valve and a proportioning valve for the rear brakes.
In wich way could this piece act up and cause a hard pedal?

As I don't have a distributioning block handy, is it possible to remove the rubber plunger inside behind the tall brass nut to eliminate the proportioning feature to test if this is the problem? Eliminating the proportioning should make it function as a simple distributioning block with a warning switch/valve.

I still don' t see how a valve like this could reduse pressure to the front.