The ONLY problem i could see with not having a hole in the res is... When the water gets warm it expands,, which puts pressure aginst the air and likewise aginst the system. An air hole would release this pressure. Although i dont think that the pressure if enough to matter that is the only rational explanation i could think of for such a hole in a res. unless you're just looking for another possable place for water to leak out of.
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