If the cap on the carbs seal properly, then pressure inside the carb top is equal to pressure in the carb bore, in which case there can be no "piston effect" as you describe. That hole in the slide is actually a fluid drain to remove to allow condensate to escape.
That slide sanding expedition you went on wasn't something I was real excited about either.............. Which pretty much ;eaves us at me asking you what you are smoking? :biggrin: