What's really nice is it's got built in hysteresis and an adjustable setpoint. I teamed it up with a vacuum accumulator. If you don't know what that is, it's basically a tank that stores vacuum. Why do we want to do that? The tank allows you to push the brake pedal 4-5 times before the pump has to run again for a few seconds. The whole goal here is to have the vacuum pump run as little as possible because it draws up to 15 amps, and the less it runs, the more efficient my car will be.
So here's my accumulator. It's got two 1/2 inch barbed fittings that I threaded in with a pipe thread that I tapped into the end cap. Then that black thing in the middle is the vacuum switch. To seal it all up I used some silicone. At one time I figured out how much of a tank I needed, but that calculation is long gone. So I'll just have to let you know how it works!
Here I've got it installed in the car. Attached to that lower tube I've got a relay zip tied on. The reason for that is the switch is only rated for a mere 10 amps, and I don't want it to burn up when the pump kicks on. So the vacuum pump drives the relay, which in turn drives the pump.
Here's another view. That bulge in the other tube is a check valve. The reason for this is when the pump turns off, you don't want air to rush back into the tank. The valve only allows flow from right to left, out towards the pump.
After I got it all hooked up, I tested it out, and naturally it wasn't wired right. The switch has a great feature that allows you to hooks something up normally on or normally off to suit your needs. Of course, this just means I get to swap wires and try again.
There's a knob on the top that changes the load on a spring inside the switch. This causes the switch to flip at a different pressure. The pump is supposed to be adjustable between 1.5 and 28 inches of mercury (FYI, a perfect vacuum is around 30 inches of mercury). I set it in the middle and tested the brakes. Still a little soft, so I crank it out a bit. Ah, much better! The pump goes back on after 4 pushes on the pedal, so that seems good, and it only runs about 3 seconds before it turns back off again. That's really good! It should reduce my overall power consumption by 3-4%, and give me a corresponding improvement in range. Maybe someday I'll actually test that...