Hello this is my first time posting, I just picked up a 73 road runner, it had been stored in a barn for the last 15 years, I have had it running already, the problem i have is, when you start it the steering wheel turns to the left quickly, by its self,!! I belive one of the valves in the power steering device is stuck. What do you think, and how would i go about fixing this, I have thought about adding marvel mystery oil in the unit to free it up. but I have not done this yet becues i dont know if it would work  

Your problem may be thick old fluid. Drain the fluid by taking the line off at the pump return. Drain the fluid into a container. Remove the coil wire to prevent the engine from starting and turn the engine over a few times to pump the fluid out of the pump and lines. Replace the line fill with new fluid and replace the coil wire and start the car. Move the wheel left and right to distribute the fluid and cut the engine off. Check fluid and add if needed. Start the car again and continue to turn the wheel right and left. The new fluid may take care of the problem. I would probably drain the fluid again after about 100 miles to get the rest of any moisture and old grime.
If the problem persist try placing the front of the car on jack stands. Make sure emergency brake is on. Start the engine and continue to turn the wheel full right and left a few times. This sometime releases the pressure from the valve and lets it work freely. If not a rebuild may be necessary. I would suspect some seal leakage after setting for 15 years.
Good luck,
     Richard