[RichardJ]

>> and violently slam on the brake pedal, <<

Where do people get this internet nonsense about jumping on the peddle to fix some problem?

The piston inside the Pressure Differential Switch will not move if the pressure on both ends of the piston ARE THE SAME.

If there is 20 psi on one side and 20 psi on the other side, the piston will not move.
If there is 800 psi on one side and 800 psi on the other side, the piston will not move.

IF, the piston has already been pushed to one side because of a previous and now repaired problem and you put 800 psi on one side of the piston and 800 psi on the other side, THE PISTON WILL STILL NOT MOVE and the light will stay ON.

You must manually and intentionally remove some of the pressure on one side to alter the balance.
The problem is deciding which way the piston needs to be pushed.

Let's say you previously had a problem in the rear. The higher front pressure would have pushed the piston to the rear. To correct this after the repair, you must crack a front bleeder and SLOWLY use the now higher rear pressure to push the piston back to center and turn the light OFF.

DID I SAY SLOWLY? No jumping.

If you jump on the damn pedal, all you will accomplish is to push the piston past center and all the way to the front. The light will still be ON. You will be standing there scratching your head or your arse, don't know which one and won't know where the piston is.

>> I have pushed and pulled the pin but it does not move. <<

The pin on the front of the Combination Valve is part of the Metering Valve. If the pin will not move in or out, The Metering Valve is gummed up or damaged in some way.
Remember that the pin movement is different for the Metering Valve in the different Combination Valves. On some types the pin moves in and on others it moves out.
Be sure you know which style you have, before you try jamming that pin in there thinking you can somehow push the switch piston and turn the light off.

Summit sells replacement Metering Valves that CPP uses in their Combination Valves. As you can see, there isn't much to it.

They don't say at what pressure this valve opens and my guess is that if you ask CPP, they wouldn't have an answer. I don't think the pressure is really that critical unless you have an old soft riding barge with a real nose-dive problem.