[RichardJ]

>> holds the "bleeder rod" to center the "pressure differential valve" piston.<<

The pin sticking out of the metering Valve (bleeder rod), has nothing to do with the "pressure differential piston". Moving that pin in or out DOES NOT move the pressure differential piston.
I said "in or out" because on some combination valves, the pin must be held "out" when bleeding. This tool is a spring clip that holds the pin "out" for that type of combination valve.