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Heater hoses should be connected to top of manifold and to water pump. Doing this creates a path from near the thermostat back to the water pump which allows hot water to enter the top front of the manifold where the 'stat is located Without that path, or with it blocked, the thermostat may not "see" enough hot water to open. Many of the old trucks like ours have a shutoff device to prevent water from circulating through the heater. If you drill a small hole, about 1/16", in the thermostat it will allow some coolant to circulate through the radiator even when the 'stat is closed regardless of flow through the heater. This will bring hot water to the sensing element of the 'stat.

I have seen a number of the Stant Superstats that act as you describe even with proper flow through the heater. I now opt for the regular Stant or more often, for another brand.

A question for you: Are you filling the radiator right to the top? If you are then you have no room in the system for coolant expansion. This will cause coolant to be pushed out the overflow line no matter how well the cooling system works. The radiator should be filled to about 1 1/2" below the filler neck. Fill to the top works if you have a recovery bottle and a cap designed for a recovery system.