Oil leak at the power steering tank

I have a Zetor 12045 1985(approx) with oil leaking out of the safety valve of the power steering tank. I have limited experience with tractor and hydraulic system.
The oil is coming out of a gap between the tank and the screw of the safety valve. The large nut on top is also very loose so I am wondering if there is a part missing out of this assembly. I can easily move the valve up and down without any resistance.

First , check the tightness of the 2 big nuts on either side of the valve, also vibration can cause the pipes to crack where they go into the nuts, so this needs checking, if they are cracked they can be brazed quite easily.
The big nut on top just screws down onto the tank, which is threaded to take the valve , so do it up tight by screwing it down onto the tank, there isn't any pressure on this unless the valve is damaged. Remember that the tank should be filled no more than half full, and use ATF oil to make sure it doesn't foam.
Hope this helps, Dave