I’m wondering if anyone knows the name of this valve. It’s on a hydraulic steering setup for a trailer. It seems to be a one way fill valve but I’m after the adapter to fit in it to fill the system.
A photo that shows almost nothing of the whole unit and application, is like setting a tiny ant on the roof of a house and asking it what are you standing on, a house or a shop.
Going by your description, maybe it is called a trailer steering valve?
To fill the system, why not look for the tank that contains the hydraulic fluid, and a fill point there?
Have you thought of contacting the trailer manufacturer?
It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)