Ok, how about this.

In order to keep air from leaking into the system, you need to devise a way to have fluid continually running through the system. So in order to keep this flow of fluid, you need a hydraulic pump to flow the fluid, but you also need a way to get fluid in and out of the cylinders.
You would have to install an additional nipple fitting at each end of the cylinders.

You would have the normal line that went between the Master and slave cylinder, but you would also have a "feed" line on each side of the piston on the master cyl. With the pump running, it would pump fluid into the slave cylinder, up the line that runs to the master cylinder and then out of the additional nipple fitting on the master cylinder back to the fluid reservoir. (this would make the fluid flow start at the lowest point of the system and work any air up to the highest point of the system (the reservoir)).

With the pump running fluid through the line constantly, it would always be forcing air out of the system, but since it would be operating on both side of the cylinders, it wouldn't steer the airplane any. The only issue I can see, is it possibly making steering the airplane a bit harder due to the fluid moving.

Thoughts?