I am interested in finding a spring check valve that will allow me to place a drain approximately at mid point in elevation on a water system. The operating pressure is less than 100 psig. I am looking at a Watts series 600 bronze check valve. The purpose of the valve would allow air in while draining to vent the system but not allow water out. It would be used in conjunction with a ball valve on the upstream side. The valve needs to be bronze 3/4″ NPT. I don’t know the cracking pressure I would need for this valve and the literature that I have doesn’t tell me what the cracking pressure is on the valve I have chosen, so choosing this valve is a shot in the dark. Can anyone help? Thanks, Jim