What would keep the coolant resevoir from flowing to the radiator? on 2002 Buick LeSabre

The coolant will only flow to the radiator when I remove the radiator cap. The engine is losing coolant but there is no evidence of a leak in the hoses or the radiator. I suspect the intake manifold or head gasket. The radiator is bone dry after even a short trip to work and back (10 miles). The resevoir remains full until I open the cap on the radiator, then the coolant flows freely.

Asked by for the 2002 Buick LeSabre
have the cooling system pressure tested at a good repair shop
2 more answers
Don't drive it .recommend towing it to a reputable shop for proper diagnosis. Sounds like it might be best to see what it's doing first-hand.
Ziptie and Global Helper are correct to not dive and have it checked.
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