The pressure differential valve is the device that alerts you if you have a leak in one of your brake circuits. The valve contains a specially shaped piston in the middle of a cylinder. Each side of the piston is exposed to the pressure in one of the two brake circuits. As long as the pressure in both circuits is the same, the piston will stay centered in its cylinder. But if one side develops a leak, the pressure will drop in that circuit, forcing the piston off-center. This closes a switch, which turns on a light in the instrument panel of the car.
This setup #1, #2 is a pressure differential valve and switch on 1976 cars. It senses that there is different pressure in one set of lines vs the other. e.g. brake failure
The purpose of the switch (item 2) in the brake line manifold/valve (item 1). When this switch detects enough of a pressure difference between the redundant front circuits, it latches on, and that turns on the dash light. It stays on until you reset it manually.
You can easily reset the switch by pressing the little plastic plunger between the electrical terminals of switch #2. The plunger may be covered by a rubber boot, and the whole thing may be covered in stealthy grease and dirt.
I was more curious to see innards of the unit, so I was able to find one from FAQ member. I bought ¼” pipe plugs (x3) and plugged 3 ports and connected 4th port to a grease gun. After a few strokes of grease gun and rotating spool end a few times was able to get the piston out. 
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