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On top of the C4 is supposed to be a vent cap, right?

I had one once that did not have a vent. My current tranny does. I also have an o-ringed, locking dipstick. When working on the car this weekend, I drained the tranny fluid twice. I have a deep aluminum pan with a drain plug. When I pull the drain plug, I can clearly hear the tranny sucking air in through the vent hole. I was surprised to hear the noise so clearly. I'm wondering if there is any problems that might arise from not having the tranny vented enough.
 

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All C4's were vented - the early C4's had a vent just above the servo and it was a 1/4" tube tha came out of the tranny, made a 90 degree turn toward the back and then just to the rear of the servo it made a final 90 degree turn down.
You are probably hearing the fluid sloshing and not air being sucked in through the vent. The vent is there to allow air to escape from the tranny as the oil heats and expands. There is very little exchange that actually takes place. There is a baffle directly under the vent and as the fluid is flung around it hits that baffle. That can make a sound like air moving under pressure.
Paul
 
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