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As soon as the rpm drop .
To answer your question, yes if you squeeze the lower hose you will def feel a spring if one is inside . I didn’t have a spring when I put mine together, but if I had one you can bet I’d put it in there. Cannot hurt.
Yes, you should feel it. Do you need one? Starts arguments. People have long said the hose can collapse from water pump suction at high rpm, but makes little sense since the top pressure in the radiator runs say 5 to 16 psi above atmospheric. If it collapsed to cause poor coolant flow, pressure would certainly rise near the relief pressure. Seems the only way to get the lower hose below atmospheric would be if the radiator tubes were clogged to cause a >10 psig drop. You also get ~2 psi increase at the bottom from the "head height". But, just to be anal and in case my engineering schooling was wrong, I put a spring in my cars. Usually the factory one is rusted away, so I wrap stainless tie-wire from a roll I bought long ago at Harbor Freight.
My spring (from cjponyparts) arrived yesterday. I'll get it installed when the weather allows.