From the link I posted, they were talking that they were reliably firing the solenoids with 9vdc batteries, so I would wire it with a spike to the 12v battery. Let it run, and monitor how warm it gets, just to watch for early failure.
Not sure how much to spend on an analog pressure sensor, but we use these in industry.
http://www.ifm.com/products/us/ds/PN2024.htm with good success.