To 5 or better
I have received you email and this is rather one lengthy explanation to your cooling system:
The radiator? Imagine your cooling system is a tea kittle filled with water. Now put it on the stove and invite us for a tea party. Since the spout is open, the pressure remains constant at atmosphere, but when the water temp goes up; the water will expand (more volume)-- HS chemistry stuff—and will pour out. But you reach a point where water starts to boil at about 212F (atmosphere dependent variable)
Now to simulate your case place a cap on the spout of the tea kittle where it can hold up to 10-15 psi. You will witness some increase in pressure which will exceed the cap rating permitting some (not all) water to escape. Keeping the flame under the tea kittle, water will not boil even when you reach the 212 F mark. For now let us ignore the fact that Coolants DO raise the boiling point of water. That is one of the reasons why you must have (antifreeze in winter and summer)!!
The overflow tank? Escaped water above is channeled to this tank by the hose you mentioned as excess leaking. When the motor cools down, coolant gets sucked back in by a vacuum created by the same water contracting (water is cooling now/engine is off) Radiators (unlike tea kittles) operate in a sealed environment coupled with the above vacuum; water gets sipped back from the overflow tank. The overflow tank never meant to be sealed otherwise it will collapse right after the radiator’s suction effect.
The cap? When the engine gets hot the cap opens up. Again hot water is channeled to your plastic overflow tank (to the ground in you case) When you turn your engine off a new air must be drawn into the system to replace the air you lost when engine was hot otherwise your hoses won’t like the vacuum and thus collapse. Does this sound like a bad radiator cap to you?
I hope this will help you in figuring out your problem