Earwigs, mud-daubers, wasps, bees, and a bunch of other insects are prone to climbing into the smallest of holes and making it home. When it has anything to do with a hydraulic system, usually in a vent tube that you may not even know you have (!), the symptoms are just plain odd. One might that that the hydraulic or pneumatic pressure in such situations would blow the blockage out. One would be wrong. Vent tubes are the easiest thing to clear and fix, and they're the last thing we look at, every time.
You've my condolences and empathy.
“Scientists are people who build the Brooklyn Bridge and then buy it.”
Wm. F. Buckley, Jr.