Often I'll remove a part that seems to not be working correctly and disassemble it and inspect it closely, perhaps under a microscope, looking for any visible indication that there's an actual, identifiable problem. Usually, looks good. This 6-port HPLC valve stator, you don't need the microscope to see the bright shiny bit where the coating is completely gone between two of the ports. That's where the leak is! It's supposed to work up to 860 bar. In this condition, it does not!
