I had a similar problem in 2015 Sequoia, the hot and cold lines are T'd together at the outlet of the bypass valve. both need to have a check valve to prevent back flow. Only allow flow away from the valve. Mine was missing the one on the hot line.
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2015 sequoia 38HRL
2005 F250 King Ranch Crew Cab, 4x4, V10, 4.10 gears (150K and counting)
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