Recently, I noticed that my 1986 Dodge Ram D100 Slant-Six no longer builds pressure in its upper radiator hose, even once it has been driven and reached its normal operating temperature. Thus far, I have replaced both the radiator cap (thinking that it was malfunctioning and allowing a pressure release at a pressure lower than it was designed to) as well as the thermostat (thinking that it was stuck closed) without any success. My next two thoughts are that there is a blockage somewhere in the radiator which is restricting the coolant flow, or that the water pump has failed (although there are no leaks anywhere). Am I on the right track? Anyone have any other suggestions? This IS somewhat of a concern for me as when I had first purchased this truck, pressure clearly built in the upper radiator hose (detectable by the inability to pinch the hose together) once it reached operating temperature (as do all of my trucks regardless of year or engine)......

 

The only things that can cause a lack of pressure are a leak or a bad cap. Pressure rises in the system due to the coolants expansion when heated by the engine. The pump and thermostat have nothing to do with a lack of system pressure. You may have gotten a bad cap or the wrong replacement cap.