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I have a 1970 Chevy C20 truck that's powered by an inline six-cylinder engine and a TH350 transmission. It's a great truck but sometimes when I shut off the engine I hear a "psssshhh" sound. Can anyone help me out?
via the sporttruck.com forums

This response comes not from the Sport Truck staff, but from one of the forum members at www.sporttruck.com. Lost_1 replied to 10EM28's question with the following information:

"Its sound like a vacuum leak somewhere. What you are hearing is the vacuum equalizing once the source, in your case it's the engine, is shut off. If you have power brakes then either a check valve located at the vacuum hose inlet is bad or if the noise sounds like its coming fromunder the dashboard, then it may be a ruptured booster diaphragm. It's also possible that a vacuum line to the heater/air conditioning controls is broke. If the PCV valve grommet has gone bad then that would cause the sound you're hearing along with a rough idle condition."

A good way to check is to take a pair of Vise-Grips and clamp-off the vacuum hose going to the valve, then shut off the engine and see if the hissing sound goes away. If it doesn't, then do the same procedure to the power brake booster unit (just don't drive while doing this!)."

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