Did the motor oil run out, or the trans fluid? It may also help to know what car and motor it is!
If you've got an auto, there is supposed to be a vacuum line from the manifold or carb base, to a small fitting on the back of the trans. This part down the back, is called the "vacuum modulator". Usually, the vacuum line is steel with a short length of rubber hose on each end. These rubber end hoses and steel line between should be the same as for a 200-equipped Mustang - easily bought if needed.
Your trans won't shift right if the line is not connected to the modulator and engine, and otherwise airtight. Nor will the motor run well, with a tube leaking air into the inlet manifold.
The PCV valve hose is much fatter in diameter than the vacuum line, and goes between the carb base or inlet "log" and the valve cover. They're easy to distinguish.
Hope this gets you started. Post back if you are having trouble finding where the hose plugs into the trans or motor.
