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vacuum check valve, if you had a leak all the time it would never come out of the vents, the check valve holds vacuum from leaking back into the manifold when under load, when it goes bad under load(while hitting the throttle) it take the vacuum away from your control panel causing it to default in the defrost, one of the vacuum lines coming from the manifold will have a check valve in line, probably the one going to the vacuum resivor... check it out, you can test it by applyin vacuum on both sides, one side should hold vacuum, one shouldnt...
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