I would replace the capsule in the mono valve. That sounds like the problem to me. My wifes '84 TDT did the same thing and it was the valve capsule. It is a very easy repair. Also, do the lines to the mono valve get hot when engine is running? You should also ck the tube to make sure it is not clogged from the water pump to the valve. Ck the outlet at the water pump, it can clog also. Ck that you have flow thru the heater core. take the hoses off and run a water hose,at low pressure, thru it to see if it is clogged. Ck voltage at the mono valve with a test light. It will glow dim if it has juice to it I'd bet on the mono valve being faulty.
Chris

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My Truck.. 2007 DODGE, 5.9 Cummins, 6spd stick, 4X4. My car..1977 240D, OTHER WHEELS...1955 VW Oval window bug, European Delivery (Holland) with a 1700cc, 2 barrel, Porsche drum brakes. 1939 WILLYS Pick-up. 1967 Triumph 200cc Tiger Cub. 1976 Honda 550F 4cyl Motor Cycle.