[911monty]

Were you able to positively identify the purge valve leaking? You said air but should be steam? When it is actively leaking you should be able to see from where. The purge valve is not supposed to leak externally. If it does it will need to be replaced. That's why I've suggested this for the 3rd time. You have replaced the cap so it can be ASSUMED the cap is not leaking. I don't like to replace parts needlessly. Is there a white crust around the purge valve. Throw me a bone here.
At this point I believe your leaking is due to heat soak. After your drive when the engine is shutdown Coolant circulation ceases and temp increases from the residual heat in the cylinders. This is normal. However if the system does not maintain pressure such as the purge valve leaking, the coolant will boil, turn to steam and leak. The purge valve can be replaced, but it is more cost and labor effective if it can be identified as the source of the leaking.

On reflection perhaps you aren't aware the overflow hose that you see leaking is the drain hose from the basin cast into the top of the expansion tank, that the coolant cap, dipstick and purge valve are located in. When they leak, liquid is collected and drains out that hose. Yes?