I am replacing parts to the heater hose and mine has this fitting installed by a previous owner. I have never used it in the 3 years I have had it and honestly not exactly sure what it is for. I assume flushing the system. When I got it there was green antifreeze in it so I removed the bottom radiator hose and flush the system thoroughly and replaced everything with Toyota red. Should I keep this piece or throw it away when I replace the hoses?