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You might check with GM for an updated vent solenoid,they have one for trucks.What happens is thet get full of dirt\dust and cant seal.The new style comeswith a different valve and filter.You need to get about 10 feet of 5\8 heater hose and locate filter above trans using tie wraps
"Silly question maybe, but can they be cleaned/blown out with air?"No, don't do this, they can become pressurized and explode like a hand grenade. I've seen it happen, it's not pretty.