Check the line between the vacuum pump and booster for oil. If none, it could be just the shut off valve is leaking. If there is oil there too, then it will be worth it to rebuild the vacuum pump as well.

That can also be why it takes a while to shut down. The oil while flowing is also plugging the hole enough to allow the vacuum to pull the shut off valve diapragm...Kinda like how taking a pull on a really thick milkshake can collapse a straw, while still getting the milkshake out...

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RRGrassi


70's Southern Pacific #5608 Fairmont A-4 MOW car

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