Stock '78s do not have electrical headlight relays. The little foam filters in the headlight vacuum system relays can deteriorate and/or fall out. They will then ingest crud that can look and act like corrosion and prevent the "dog bone" valve from sliding and properly directing vacuum to the headlight vacuum actuators.
First, check the integrity of the diaphragm in the top (disk-shaped end) of the relay by attaching a vacuum pump to the SMALL port extending from the metal "hat". If it does not hold a vacuum, the diaphragm is bad and the relay cannot be repaired. If it does hold vacuum and the crud or corrosion is preventing the "dog bone" valve from sliding and correctly porting vacuum to the actuators, repairs may be possible.
Before disassembling the relay, you might try squirting WD-40 into the 3 LARGE vacuum ports and into the bottom end of the relay after removing the filter material, if any. This has been successful for some in freeing up the "dog bone".
My sticky thread, "How Your Headlight Vacuum System Works
" includes info and links to more easy relay tests (and other headlight vacuum system tests) you can perform and links to a couple of fairly easy "dog bone" valve repairs that have been reliable and saved some $$$.