On my Triton TR-140, I have a (roughly) 1/4 black hose that is inside the filter touching the filter top. The bottom of that hose is buried in the sand and connects to the center pipe from the laterals to the pool. Triton refers to it as an "air relief tube" or an "automatic air vent".
I do not understand the function of that tube. I assume it is not absolutely essential as it could be fairly easily submersed in the water in the filter and blocked completely. Any ideas?