The vacuum kit I ordered for the car arrived today.
Most cars on the road have power assisted brakes. When you press the brake pedal, a brake booster increases the pressure in the brake lines in proportion to the force applied at the brake pedal. The brake booster operates on vacuum, which is available from the intake manifold on internal combustion engines. On an electric car, the vacuum must come from another source – a vacuum pump. On some electric cars, the vacuum pump can be the noisiest part of the car. I found a pump kit that has a reputation for quiet operation: VBS-EV-12 . The pump operates from the car's 12 Volt system. The exhaust of the pump is coupled to an oil filled muffler. This cuts down emitted noise. The pump will be installed with a rubber isolation mount, to keep vibration from transmitting through the structure of the car. If pump noise still proves offensive, then the final contingency is to mount the pump in an acoustic enclosure.
When the vacuum pump is switched on, the pressure in the brake booster drops. Once the proper pressure is achieved, the pressure switch will open and turn the vacuum pump off. Provided there are no leaks, the vacuum level can be passively maintained until the brakes are operated. The check valve prevents the vacuum from leaking through the unpowered pump. After a few cycles of pressing the brake pedal, the pressure will rise, and the vacuum pump will switch on. The purpose of the buffer tank is to store vacuum, and increase the number of times the brakes can be operated before the vacuum pump turns on again.
The electrical circuit will only turn the pump on when the pressure switch measures poor vacuum, and when the ignition switch is in the run position. The main relay is rated for 40 amps. The reason for the second relay is not clear to me. The only thing that comes to mind is that the coil of the main relay must draw a fair bit of current, and the second relay keeps the current drawn from the ignition switch reasonable.