Drum Brake Pressure Switch

[RichardJ]

The brass piston shown has square shoulders and makes contact with the steel pin. The switch at the bottom of the photo would be mated to a piston that has tapered shoulders. The taper pushed the nipple on that switch, back into the body of the self contained switch.
I've not seen the one Loudchevy has pictured. It appears to be just a slight variation of the one I have shown with the long thin steel pin. The protrusion with the slot appears to be electrically insulated.

If yours needs cleaned internally, it isn't that difficult to disassemble. If you are careful, the sheet metal screw will do little damage to the brass inserts and can be reused. There is nothing in there, but two springs and the piston.
In this picture, there is a residual pressure valve below the seat, but you won't have that. They're showing seat removal for a MC, which may or may not have a residual valve.
Push the piston out with drift pin and clean with solvent and flush with brake fluid.