I'm restoring a 1985 Chevy G30 Van 5.7L, but these questions apply broadly to carbureted vehicles.

The vehicle is carbureted, and most controls for the emissions system and climate control system are vacuum operated. The vacuum source for all systems is underneath the carburetor, or half way above its throttle-body, or from the intake manifold, depending on which vacuum circuit.

It seems to me all these vacuum lines are exposed to gasoline vapors from these vacuum sources. Many of the lines are rotted after so many years, and I've replaced some of the smaller 5/32" (4mm) rubber vacuum lines with silicone. However, I know silicone has notoriously poor gasoline resistance. I am worried that the gasoline-rich vacuum will eventually eat the new silicone lines. I know, for example, that the larger 3/8" lines for the PCV need to be fuel-resistant (oil and burnt gasoline crankcase fumes)

My first question is: are vacuum lines in carbureted vehicles exposed to gasoline vapors generally? If so, is it enough vapors to eat through silicone? If it is only specific vacuum control circuits, how does one tell which lines should be fuel-resistant? Maybe just the vacuum lines closest to the carb / intake manifold?

Also, vacuum systems in general should not be vented anywhere to the open atmosphere correct? There are some stiff, thin 2mm nylon lines that go to nowhere (vented to open air) from the vacuum reservoir. I'm trying to find where they go, but I'm also wondering if there are ever lines like this to slowly vent out the gasoline vapors from the vacuum circuit.

Here is a picture of my vacuum diagram for the emissions system.

emissions system vacuum diagram for 1985 Chevrolet G30 5.7L

I can't find a vacuum diagram for my climate control system, but I believe it is similar to this:

enter image description here

1 Answer 1


Many of the lines are rotted after so many years

On a vehicle of that age, I replace every single rubber line without exception. I use shrink tube or paint to replace any OEM color markings on the hoses or to correspond to diagrams.

I've replaced some of the smaller 5/32" (4mm) rubber vacuum lines with silicone.

No. Just No.

Do not "freestyle" or "crossover" one type of line for another based on experience you have in other crafts. There is only one correct type of line, and that is automotive vacuum hose.

The operating conditions of an automobile are extremely demanding. Quality automotive materials and components have evolved for 100 years for that demanding service. If "reliability" plays any part in why you are replacing those lines, then use the correct materials.

I don't even stock the stuff. I just walk down to Autozone or O'Reilly and buy a few feet when I need it. It's cheap, even there.

A Chevy truck/van is possibly the most supported vehicle in North America, certainly of that age. Most stuff is available except for a few smog parts.

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