This article is aimed at buses 1971 to 1979 that have dual carb set ups. The big problem is the brake booster. If you have two one barrel set ups then most guys just “T” into that and that provides the vacuum they need for the booster.
I am not a huge fan of the balance tube that you're “T-ing” into so what I do is to run one way valves in that balance tube to keep vacuum from robbing from one side to the other. So the set up is, come out of one side with a one way valve. The other side with a one way valve then into the “T”. This seems to work the best for me.
What if my manifolds don’t have a port for the vacuum or the port is too small for hose I need to use? For the first, you will have to drill and tap a fitting or weld in a tube for the hose. When doing this make sure the fitting is level or pointing up. If you point it down at all, gas will get into the hose. Not a good thing. If you are looking for the one way valve the stock ones are available. That is what I have used.
Now the biggy- I am a huge fan of the IDF style dual carb set up. That is; two, two barrel carbs. Getting these to work with the booster is not an easy deal. I have tried all kind of ways to get the booster to work with little or no success. Fist way I tried was to tap into one intake runner. Yeah that didn’t work. The booster worked ok but that cylinder ran different than the other three. That was out. So then I tapped into all four cylinder intake runners. Put one way valves on each hole. Had lots of “Ts” to join them all together and a lot of flippin hose. Does it work? Yeah it did, but man it looks like shit and leaves a lot of areas for problems to occur. The motor wasn’t super happy either. Keep in mind you have to have a one way valve on each runner. If you don’t, you just turned your individual runner system (which is a damn good system) into a plenum system. I have done this system to a few motors and I have to say I was never really happy with the results.
With that said, I started looking into a better and cleaner way. What I came up with is an electric vacuum pump. The pump I used pulls 20hg of vacuum and draws 6 amps. I had to fab some adapter bushings to up the size of the hose to 12mm. At first I put it in the engine compartment and used a relay to power it. I soon found that it wouldn’t shut off. Come to find out, I had leaks on the OE hose that went front to rear. I could have replaced all that hose but decided to relocate the pump up front just behind the front axle beam. Now my line was about 2 ft long. I had to route the hose away from any moving rods or cables as to not rub a hole in it, but that wasn’t too hard. I again used the relay. I had to work off of keyed power. That way it would only come on when when the key was on. It worked great. It does make a little noise when pumping, but nothing I couldn’t live with.
All and all I feel this is the way to go. It’s a lot less work than adapting the intake manifolds and all the ugly plumping. The motor is not getting robbed of any vacuum and that makes me happy and makes the motor a lot happier too. The installation is clean and it works well. Cost wise I think I have about $350 into parts. Not cheap but a great way to go
