my 1990 had a high pressure air powered vacuum pump in front of the drivers side headlight. it used air pressure to move a diaphram that made vacuum and the low pressure vacuum line ran up through the floor to under the dash to a black ball tank that was the vacuum pressure tank for the vent and defroster controls in the plenum. there is a by product of air that blows off of the vacuum pump.
mine sprung a leak and i replaced the whole thing with a 12v pump off of a f350 diesel pickup truck.
from looking at the picture i suspect that is what it is.
the reason for cycling would probably be the vacuum pump is detecting it needs vacuum so it turns the pump on...
i am reaching but it kinda works as an explaination.
tom
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