Could be a combination of factors.
If one of the "up diaphrams" is bad it creates a vacuum leak. That will prevent that headlight from closing. It may or may not prevent the other one from closing. If you have good vacuum the other one may still close, but slowly. If weak vacuum, it may not close either.
Could be a leak in the line from the switch that goes to the "up diaphram" or could be a bad switch.
There will be a T in the hose to the "up diaphrams". You can pull the supply hose (from engine/tank) and check for vacuum on that line. You could also hook up a MitiVac to that T and see if the the "up diaphrams" hold vacuum. Then use vacuum/MitiVac to test each diaphram individually.
I have found that the original diaphrams are VASTLY superior to most of the aftermarket diapharams. About 10-15 years ago I had a center diaphram go bad and had to replace the actuator. I figured the other side was probably not too far behind and I almost replaced both of them at the same time. Glad I waited, as I had to replace the diaphrams on the replacement actuator 2X before my original (or replaced decades ago) diaphram finally failed. From my experience, if you get 5 years out of a repo diaphram, that doing pretty good.
I have now converted to an electronic setup. It was quite a bit of work/fabrication, but seems to work very well.
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