Your photo shows a disk drum master cyl.
The front most cyl outlet has a valve in it behind the seat. That should be removed for the rear disc to work correctly.
As for the rest of the system if you get fluid to the rear calipers than there is no good reason why the rears do not apply.
When you say the rears do not apply, how are you determining this?
Are you sure you have bled all the air from the line? Maybe try the old fashioned method of 2 people and a bottle of brake fluid.