radiator cap

Hello, my question is the next the blush radiator plug, the spring is broken and it no longer works and I do not know how to ask for it since it has no numbering indicating the pounds of pressure that is required to make it normal could help me to tell me how many pounds of precion it has to be the plug. The Pick Up is a Dodge RAM 50 Motor G53 Model 1988 by hand thanks for the answers