I once owned a 1968 and a 1969 LTD Brougham with those "Hideaway Headlights" and they were vacuum nightmare. The 1969 had a manual rotary "switch" to disengage the vacuum and leave the covers open.
Like Adam suggested, it's likely a cracked or split in the vacuum tubing somewhere... especially with tubing that is probably 40 years old. Hopefully, it's the tubing (which is replaceable) and not a vacuum diaphragm which might not.
Sig removed. Read the rules.
So, I read the rules. Now I'm even more confused.
My transportation sources:
1972 Green Schwinn Speedster; 1984 Canyon Red Mustang V-6; 2004 Gold Ash GMQ LS Ultimate Edition, (Traded away, sadly); 2004 Black Ranger XLT Flareside; 2019 Turd Brown Fusion Titanium Hybrid Energie.