You're suggesting it was never open to the outside or had another way water could get in.
There are two stone blocks in one of the shafts, and the pyramid itself had casing stones.
I'm not saying that the shaft was never exposed to the outside but I do not think it had a literal function, nor one involving induction or expulsion of water.
Maybe carbonated water fizzed up and out of the shafts. What does your translation of the PT suggest?