In a conventional car the engine generates a vacuum in the intake manifold as the pistons pull air into the cylinders on the intake portion of the cycle. This vacuum is used to provide force on a membrane inside the power brake booster, to reduce the amount of force required on the brake pedal. Electric cars (and diesels, and hybrids) don’t have this built-in source of vacuum, and must generate it with a mechanical or electrically driven pump.
I used a popular option, a 12VDC powered diaphragm pump manufactured by Thomas. This pump is often used in portable medical equipment and ambulances. It’s installed on the side of the “mid-mounted” battery rack with rubber isolation mounts. Symmetrically opposing it is the vacuum reservoir, made from a length of ABS sewer pipe. I drilled a hole in the ABS, heated a brass hose barb and threaded it in, then removed it and reinstalled it with silicone RTV goo.
The pump is switched on and off with a SMC digital adjustable switch, with programmable cut-in and cut-out points. It’s positioned up front, next to the booster. The tubing used throughout is a stiff flexible polyethylene tubing in 3/8″ ID. The SMC switch has a low current transistor output which I used to trigger a higher current transistor (2N2222) to switch the coil of a “standard automotive style 40A” relay. Remember the coil suppression diode!
The pump works fine and generates adequate vacuum. If you pump the pedal deeply and rapidly you can use the vacuum at a faster rate than the pump can supply it and the power assist will wane. Fortunately that situation is never encountered in actual driving.
Unfortunately the pump is rather loud and annoying, particularly installed so close to the passenger cabin. I installed it early in the conversion and probably should have waited until later and put it in the engine compartment so it would be a little less noticeable from inside the car. Since installing it a certain taiwanese rotary vane vacuum pump has become available that is quieter and evacuates air faster- it would make a nice upgrade.