Below is a vacuum reservoir that I built. It's designed to store extra volume of less pressure air. Typically an ICE (internal combustion engine) will generate approximately 10-20 in/Hg and in large volumes so there isn't a need for this as the existing reservoir will hold plenty. With an EV we need to be as efficient as possible. This involves using a somewhat small and efficient vacuum pump that will build up the vacuum over time. When the brakes are needed the reservoir will be able to produce multiple uses of the brakes before becoming depleted at which point you'll loose the power assist and it's much harder to depress the brake pedal to stop.
The reservoir can be purchased but it's simple enough that most should create their own. All of the parts are easily found at a hardware store in the plumbing area. I used 4" ABS and the unit is about 8" to 10" long. In order to only have to deal with one hole that needed to be sealed I opted to use tees outside the reservoir. The one hole was thread with a tap and die set and a threaded tube was threaded using plumbers tape to help seal was ran through the top. On the inside a coupler was used allow me to really tighten things down. Plumbers tape was used on all connections to help avoid any leaks. The barbed fittings allow for hose to be slipped on and then using a ring clamp you can secure them well.
I made my own bracket that would take advantage of the existing bolt holes in the frame and would allow me to fully mount the complete vacuum system in one shot. I believe this was
48"x 1"x 1/8" flat steel and cost about $4 with metal leftover.
Here you can see the Gast vacuum pump secured to the mounting bracket. In addition you can also see some rather large ring clamps that will hold the reservoir in place.
A shot of the reservoir attached to the bracket.
Here is the assembly attached to the car. It's hard to see but all the vacuum lines have been attached. There isn't a special vacuum line. Anything that holds up well to the elements will do as long as the walls are not too thin in which case it could collapse under the vacuum. For all the major lines I'm just using a 3/8" ID black fuel line I found at the hardware store. You can also see a very small clear line. This runs over to the switch which is mounted to the back of the bracket. I forgot to take the picture of the switch being mounted but you can slightly see it where the red wire has the yellow shrink tubing sticking out. One of the barbed fittings was removed and capped before installation and after I had calibrated the vacuum switch.
To calibrate and test I hooked the hose my new vacuum system to the cars original reservoir and connected the battery directly. First I adjusted the switch until it turned off at 20 in/Hg. The pump is rated for a maximum of 25 but that usually means it will get pretty slow and take longer to reach that using more energy. Next I counted how long it took to bring the system from atmospheric pressure to 20 in/Hg ~25 seconds on average. Next I did multiple runs of pumping the brakes quickly to deplete the system. I consistently get five well assisted depressions of the pedal followed by two that were slightly stiffer and then after that it's much harder to press the pedal and the vacuum gauge shows depletion. Next I checked what happens after just depressing and releasing the pedal one time. Every time the pump would turn back on and take on average five seconds before shutting off. At first I was thinking this wasn't good and would prefer it to go two to three times before kicking back on but then realized a potential hazard with that. If I only have five good pedal depressions and releases and say three are used up what happens if I needed the brakes a few more times in a row in some emergency situation? I realized it turning on for a short while after each braking was really a good thing. Probably the only way to get away from this would be to have a HUGE reservoir where the pressure wasn't affected so much with each braking.
Here is a shot showing the line running up to the original brake reservoir (it's the new shiny black hose.
I still have to run an ignition lead to a relay near the pump so that it only runs when the key is on and I'll be using the wiring which was already in the area for the old pump. I'll be holding off on this until I install the rest of the electronics.
As you can see the overall system is very easy to build and straight forward. I used a Gast vacuum pump from EV America ($225). The switch was ordered from EV Parts ($23.50). Everything else was obtained from the local hardware store including the metal to build the bracket.