'97 Coolant recovery question

When the engine is hot, excess coolant transfers to the coolant recovery tank. When the engine cools (overnight), the coolant does not return to the
radiator. When I remove the radiator cap, I can tell I'm breaking a vacuum. Is this a radiator cap problem? - Mike
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It is a possibility... could also be a restriction in the hose to the recovery tank...

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Well, beside hearing the vacuum when I release the cap, the upper radiator hose has collapsed from the vacuum. Thinking it was weak I replaced it (along with the radiator cap), but the symptom continues.
I removed the small line from the radiator filler neck to the recovery tank and the vacuum remained in the radiator.
Open to suggestions - Mike
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