Question #1: The sand leaked less before I replaced the sand. it is definately worse now, but it was always leaking since i bought the place.
Question #2: The sand leaks when the pump is on. I replaced the gaskets on the purge value to ensure i wasn't seeing sand leak in from backwashes.
Question #3: It is a Sta-rite HRPB30 ; the air vent is a small metal tube at the top with a metal valve on it.
Question #4: this is the pump/filter combination i had when i purchased the house. The pump is a sta-rite. I expect I am getting about 80 gpm with the setup, which is inside the specifications for the filter (according to the owners manual, at least).
Question #5: The manual called for 400 lbs of sand....and that filled up half of the bell (to where the bolts are)
Question #6: The sand was labeled as 'pool filter silica sand #20'. I tired to find the sand the owners manual said to use (45mm-55mm) but could not find sand graded that finely.