There is a membrane behind the black plastic end cap and that membrane can be damaged for instance when too much oil is put into the system. Since the membrane is a part that cannot be damaged just by using the brakes as intended or bled the right way, the plastic end cap cannot be removed and therefore the membrane can't be replaced. This is probably why your brakes are not working.
Kind of starting to wonder if the hole you see the leakage out of is actually another "port" per-say to extent the housing for a drop bar addition like in this picture... Have this setup on my cycle-cross/road-bike.
Hope this helps!