Ford vehicles can be tough to bleed and air can definitely leak in. The likely leak spots are on the suction side of the system. This includes the front pump seal, particularly if a rebuilt pump is used, the suction hose connections and the rack and pinion.
When running only the pressure side of the system is highly pressurized and the return side is not. This can cause air to enter, without leaking fluid. Finding the leak can be difficult, we use a special device to pressurize the entire system, without the pump running.
If there are no leaks, it could be air is deeply imbedded in the system. We often have to attach a vacuum pump to these systems to extract all air. Here is an article on the topic that might help.