How do you bleed the air bubbles from a hydraulic tilt and trim on a 1978 evinrude?

The trim system on Johnson / Evinrude outboards, like most other engines, is a self bleeding, enclosed system. Run the unit through the complete cycle, all the way down, and then all the way up once or twice. As you open the fill plug, you should hear any trapped air escape through the fill hole. If fluid is present at the fill hole, the unit is full, if not, fill to level and replace the fill plug. Also, the air bubbles in the areated fluid will settle out by letting the unit rest for a while. Repeat this process until the system runs smoothly through the complete cycle, and the oil reservoir remains full.