it is a check valve designed to keep the fluid from exiting from the cylinder too quickly in the case of a line rupture. Some mfg's call it a velocity fuse, others call it a safety down valve, cat calls it a check valve.
It may serve a dual purpose as an equalizing valve but its original intent is to restrict flow in the case of a line break/rupture. (a safety check valve)

Unfortunately when this valve malfunctions it causes raise/lower issues and mast sequencing issues as well.

so whatever it is called by the mfg it has been an issue in many cases that I've seen such as this.