The requirements for the symbols are specified in SAE AS 1290A which is adopted as a de facto ISO International Standard. For the purposes of international standardization, the amendments listed in table 1 shall apply to the specific clauses of AS 1290A noted under "Reference".
ISO 5859:1991 history
2014ISO 5859:2014 Aerospace series - Graphic symbols for schematic drawings of hydraulic and pneumatic systems and components
1991ISO 5859:1991 Aerospace; graphic symbols for schematic drawings of hydraulic and pneumatic systems and components