The Astral Air Relief Plug Kit, Plastic – Relief Plug Kit is the replacement air relief for Astral Terra filters. This Relief Plug Kit pool air relief is necessary to keep your filter running properly. If there is too much air in your filter, it will not allow water to flow through properly. The Astral 4404210304 assembly will repair it. This assembly includes both a threaded key and an O-ring. It is made of durable plastic to ensure that it works properly and keeps your filter primed.