Questions on water pressure sensor and strange solenoid?

Okay, after deciding to add an oil temp sensor to my car I'm all out of analog channels on my CDS SSI. (Currently using Lat G, Long G, Front brake P, Rear brake P, Oil P, Oil T, Water T, Throttle, Steering, GPS) I've been super paranoid about water pressure ever since I had an SM spring a leak from a rock puncture to the radiator while on the track. I naively thought my water temp would save me in this situation but quickly found out that once the coolant system was completely empty the temp sensor would then read the air temp which was still below 200 degrees for a few minutes after the system was empty of water. After rebuilding the head because of overheating I quickly learned to always have a water pressure sensor to sound the alarm in the event of a leak.



So, I decided I'm going to also add one of those longacre water pressure warning light kits to sound of the alarm when the system gets below 3-4 psi. I was going to add an inline hose adapter with a 1/8" port but saw that the cooling resevoir has an available port that that has a plug threaded into it. It appears to be 1/8" NPT and not 1/8" BSP which I assumed it would be. It looks perfect for a pressure sensor install and wondered if anybody else thought this was a sound idea?







Also, I am trying to figure out what this solenoid type contraption is on my rear brake line. It has a ground and power lead with the ground running to the chassis and the power lead going to a switch on the cockpit switch panel. What in the world would this be used for and why would it require a switch to activate?