Not something I came up with but found on the internet. There were a few different ways to make this design. From what I read most of the designs caused the trigger on the hand pump sprayer to not work anymore. This RL Flo Master Model 56HD sprayer might have been updated. Some builds show using a 3/8" FL x 3/8" FIP fitting between the sprayer and firestone ball lock post and some show using just the post connected to the sprayer. There is a washer on the inside of the nozzle that a needle presses against to stop the flow. I transferred that washer into the brass fitting at carefully tightened the fitting onto the plastic spray bottle with some thread tape. The firestone liquid post will then screw onto the brass fitting again with some thread tape. I don't have the poppet or o ring on the post in the photo.