Is there any safety issue in using a brass coupler on the end of a pressure test gauge vs a steel coupler? Our local hardware store stocks the correct size but in brass only.
Sounds like you are using a fitting designed for compressed air systems which are typically running around 110# although two stage ones used in truck repair go to 175#.
Hydraulic systems are running in the range of 2,500 to 3,000#.
Get to a specialty hydraulic shop to get a steel fitting designed for the pressure of a hydraulic system.