How do I find out why there is not pressur in the radator

I presume you have a pressure gauge installed on the radiator. and the engine is running. If you have no pressure after running the engine until the water temperature is hot, you should check the instrument of your vehicle to see if the engine is not over heating. The water pump is circulating the water so the water is in constant motion and is cooled by the air flowing thru the radiator. The air is drawn thru the radiator by the fan, either engine driven or electric powered. When the cap is off you should see movement of the collant in the radiator. also chech if you see any oil residue mixed with the water. in that case the water level should drop because the water is draining into the oil tank.. If you see no water movement the water pump is not moving the coolant thru the engine and the engine will over heat.The water pump is bad in that case.