Backflow preventers have a common silhouette that may be used in the logo. I will upload some examples
The test gauge and backflow supply also uses a silhouette of a test gauge we use in our testing. We can use it solely, with a silhouette of a preventer like in that example or not at all.
I do look the idea of a retro feel...like a gas station patch. Maybe shape the patch in the silhouette of a test gauge? There are different styles of test gauges so feel free to experiment with making it one of a kind. I don't want the shape to be construed with a particular brand of test gauge.
This loge will become part of the vehicle body wraps. So it will need to be dominant and easily recognizable in relationship to the advertising words on the vehicle.
Another idea. How about a logo with heavy letters and use two colors separated horizontally with the shape of a preventer down the length as the separation line?
Keywords you might need to research...backflow preventer, reduced pressure principle backflow assembly, backflow test gauge, backflow prevention device.