The headlight covers on my cougar roll back after about 20 minutes in most cases but sometimes stay closed for longer after a good drive. I presume the diaphragm in that round cylinder that connects to the rods that turns needs fixing. Can that be done or is a new unit required.?
There are a number of places where you could have a vacuum leak, headlight switch, check valve ,vac. tank,hose connection but the most likely would be the vac.motor , “that round cylinder” You will have to check by removing the bottom hose, block that port and push down on the lever, if solid then can be re-built if the lever slowly goes down then the rubber inside is bad and you will need another vac. motor.
Good video about the vacuum headlight actuator here:
https://cccforum.discoursehosting.net/t/wccc-actuator-reconditioning-video/13602/1
Check out all the components as Catlover indicated. A small hand vacuum pump like a Mityvac can be help full but isolating sections and seeing if the actuator holds works as well. There are two failure modes with the actuator. The most common and repairable is the seal on the actuator shaft. WCCC just posted a new video on that repair. One caution with the Mityvac you can damage actuators is you pull too much vacuum.
https://www.youtube.com/watch?v=k1UFf3ytrro