You take it to a mechanic who has any of the dozen tools made specifically to solve this problem on Ford products. What you should also be finding is the air that is compressed in the system will expand and cause fluid to overflow from the reservoir when you stop the engine. It can be real frustrating, as you're finding out, to get the air out.
The tool I have seen people have the best results with is a funnel that attaches to the reservoir where the cap goes. The fluid that blibbits out when you stop the engine will be caught in the funnel, rather than running onto the floor, and will go back into the system.
If you let the car sit overnight, the tiny bubbles in the reservoir will slowly work their way out. Part of the problem comes from the fluid in the steering gear and hoses has bubbles yet that contract under pressure when the engine is started, that makes the level in the reservoir go down, and the pump can suck up more air. Leaving that funnel in place with extra fluid helps to get the air out.
It can also help to drive the car onto a slant with one side up higher first, then the other side. That can help an air pocket get expelled that is trapped in the rack and pinion assembly. Steer lock to lock three or four times while each wheel is higher than the other one.
You may be able to find that funnel, or some of the other tools for Fords, at an auto parts store that rents or borrows tools. Otherwise head to a suspension and alignment mechanic. They all have their favorite ways of solving this.
Tuesday, June 28th, 2016 AT 3:31 PM