I have a 55 chevy heavy duty truck with a 261 engine. This is an old firetruck.. The intake manifold has been changed to a Fenton two carb manifold. There is two vacuum lines for the truck, one is connected to the intake manifold under the carb. It goes to the hydrovac and the two speed rear axle and has two check valves in the line near the manifold. There is a second vacuum line near the intake manifold that is not hooked up and this line also runs to the hydrovac and two speed rear axle with no check valves inline. This vacuum line is correct as it goes to the hydrovac and two speed rear axle. I need to know where this vacuum is connected. The vacuum line with the check valves uses manifold vacuum. I believe the second does not use as strong of a vacuum source as the manifold. I have seen where they use air cleaner vacuum as a source. I can not find any information on where to connect this hose. Thank you for your help.
http://www.stovebolt.com/ubbthreads/ Go to the big truck area (it's called BigBolts) to cut and paste this question, lots of guys with your era truck with the answers. Cosmo in the '49
The line with the check valves is the vacuum source, the other line is most likely just a breather to keep debris out of the hydro-vac and rear gear selector.
After reading the section on the two speeds in the 55 truck manual http://chevy.oldcarmanualproject.com/shop/1955truck2/index.htm Section 4 I'm thinking that one set of vacuum lines is for upshift and one is for down shift and both are connected to the two speed switch.
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