Originally Posted by Uncle Ben
Heat rises. Coolant flows from the coolest end (bottom) of the radiator into the engine and return from the engine goes into the top of the radiator. The whole system will be close to equal pressure but the top hose would allow in flow as steam is compressible while liquid is not.
Understood and agreed. The point of the Pirate article in having you put the fitting right below the radiator cap is by design that is the lowest pressure point in the system. When the hot coolant exits the engine and into the radiator via the top hose, it is at maximum temp plus the head pressure from the water pump. As the water moves across the radiator towards the side with the cap, the friction the water encounters in the tubes coupled with the contraction from cooling causes the pressure to drop. Thus why they discuss a "high" and "low" pressure side.
They want the steam to go into the radiator at this point since it is the "low" pressure point (less chance of it being trapped in the line by high pressure), it should also be the physically highest point in the system, and if the pressure builds and the cap purges, it purges the steam first.
All that said, I suspect it is technically accurate but it does seem to be splitting hairs. I'm going to go with the adapter Marco used to ensure I get more of the air bled out of the system. I suspect that is the reason the Meanie always seems to be borderline on the verge of over heating as there is probably a fair amount of trapped air in the system. The current steam vent line goes sharply downwards to the water pump inlet so it has to be not working at all or it is trapping as lot of air in the system.