[RichardJ]

>>It does not have a reset button like the ones I’ve seen<<

You've never seen one with a reset button. You have seen a Combination Valve that had a Metering Valve on the one end that was plumbed to the front disc brakes. The Metering Valve is not part of the Pressure Differential Switch and is not connected to it internally. The Switch, the Metering Valve and the Proportioning Valve function independently from one another. They are simply Combined into and share the one single housing.

The Pressure Differential Switch shown above, is a ON-OFF-ON switch.
You must temporarily reduce the pressure on one side in order to have the pressure on the other side push the piston in the switch back to the center or OFF position.

By side, I mean front vs rear. The difficulty is knowing if the piston has been pushed forward or back.

Have an assistant push the brake pedal with moderate pressure while you crack either a front or rear bleeder screw. A hose should be attached so that air doesn't get sucked back into the system.
If the light stays ON, go to the other end and try again. The assistant must not push so hard, that he doesn't push the piston back through the switch to the other end. The light may only flicker and he not realize what has happened.

Unequal pressure moved the switch in the first place. The Differential Switch is designed so that Equal Pressure does not move it.

Having said all of the above, some guys have had some luck, removing the switch and using a small pick, nudge the piston into the center position.