The electric pickup truck uses a vacuum pump to generate vacuum for the power brakes (and move vents in the HVAC system). It has a pressure switch that turns on the pump when the vacuum drops to under 15 inHg and turns it off once the pump has raised the vacuum to 25 inHg. The current system has a small 3″ by 1.5″ PVC cylinder as the vacuum reservoir. As soon as you press the brake the pump turns back on, and cycles on and off relatively frequently. I wanted a larger vacuum reservoir so that the duty cycle on the pump would be longer (it would stay on longer, but also stay off longer) and so that even when the vacuum drops to 15 inHg I could still operate the brakes several more times while the vacuum pump was working.
I used 3″ PVC pipe and end caps to build the Reservoir. I screwed a 3/8 inch hose bib (and a smaller adapter for a $25 vacuum gauge from AutoZone) into one end cap and then made sure it was air-tight by coating the inside and outside of the adapters with epoxy. After using PVC solvent and cement to attach the end caps, I also used epoxy on that joint, just in case. Spray painting it black finished the job.