There are two types of pumps and reservoirs on those.
The old style with seperate pump and tank and new style with the tank and pump all in one unit. The old style you have to fill above the engine under the hood.
The new style you fill on the side of the engine below the generator.
A 73 model should have the new style.
Which style do you have and what is the "adjustable screw above the tank"?
Both types have a filter in the tank.
If you are certain the system is not leaking externally then it means it is losing oil through the seal on the front of the pump and going into the engine.
Can you tell if the oil level in the engine is rising? Keep a close eye on that.
If it is going into the engine you will need to remove the PS pump and renew the seals in it.
On a difficulty scale of one to ten with one being changing the engine oil and filter and ten being rebuilding the transmission, renewing the front seals on the pump is about 2.5 to 3 - ie, not hard at all.
If you ask HOW to put seals in the pump I will just tell you to go buy the I&T FO-31 Manual (google it) and read that section to learn how.. The manual costs about $30 so is cheap enough.
Good luck and give us a progress report if you will.