If you want a little safety margin, find a 30 lb. Freon drum and mount it somewhere out of sight, under the bed, etc. and run a 1/2" diameter vacuum line to it through a 1-way check valve. I just use a regular brake booster check valve and grommet, fitted into the blowout safety cut in the tank with a Dremel tool. A little strap steel to mount it, and you've got a vacuum reserve tank big enough for 2 or 3 brake applications even with a dead engine.
Jerry
