 The fact that the upper hose is hot and the lower one cold tells you that water is not circulating. This occurs mostly when the radiator core is plugged or the pump is not moving water. Since you had no problems prior to draining down the block to change the temperature gauge it is most likely a marginal pump problem. The design is a class flooded pump, it will not pump air or develop any significant pressure (less than 1 psi?), it simply moves water, even when new. An internal pump inspection won't indicate much unless it is really worn. A marginal pump will not move water when partially air bound unless the water level in the pump housing is above the eye of the impeller and the more wear, the higher the water level in the pump needs to be. Being marginal in a flooded pump is necessarily excessive wear but can be minor erosion of some part the casing or other internal parts that cause the pump to cavitate enough to preclude initial pickup but is not otherwise significant. Now look at your installed pump, there is no vent on the housing and when you drain and refill the coolant there is air trapped in the upper pump housing. A pump in good shape will sling enough water in the housing to work the air out over a short period. A marginal pump can be entirely satisfactory under all loads once it becomes flooded. To that end, a work around can be to run the engine cold to hot repeatedly preferably driving the tractor up and down an incline and rough ground. But then youwill likely have the same problem the next time the coolent is drained. The best way is to drill and tap the top of the flat on the pump housing to expel air while filling. I do that on all pumps of this type even the new ones before installation. Pic is a 310 but is the same pump and circ system as your VAC. Housing is tapped 1/8" npt with a 1/8" mnpt x 1/4" fl fitting and cap for vent. If the vent works you probably won't need a new pump but it is a quicker, cheaper option then throwing in new parts, radiator recoring, etc until it is fixed. Joe
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