[mongocanfly]

I don't know for sure but they told me when I set mine up not to plug the heater hoses which is the same thing manually valving off flow will do..they said it needs the flow thru the heater hoses for the thermastat to see hot water so it will function..didnt matter cause ive got a heater but they said if I wasn't running a heater core to just loop a hose across the fittings.that would be the same as a heater bypass valve..I do know there was a member here not long ago that was having overheating problems and he had his hoses plugged with bolts and clamps..when he looped the hose it fixed his problem.that may be why your valve is slow to close is to let enough hot water get to the thermo so it will open..once hot flow is established it can be closed off....I'm just assuming here...