When running a non pressurized coolant recovery tank its my understanding that the radiator cap should be of the vented type to allow the radiator to draw the coolant back out of the recovery tank during cool down. So if its vented it will allow coolant to drain from the high points ie.
Coolant Recovery Tank Ricks Free Auto Repair Advice Ricks
Non vented radiator cap. The top of the engine into the overflow tank. Meets original equipment specifications. The 10231 on the radiator 16 psi vented cap and the 10203 on the recovery tank 0 psi cap that just prevents. This means that if you remove the cap and allow the pressure to be released then you must wait until it has cooled back down before putting the cap back on. If you do not then it will create a vacuum inside of the radiator as it cools. The radiator cap needs to be non vented.
A faulty radiator cap can cause your car to boil over at a lower temperature. When the radiator cools a sufficient vacuum may form to draw air back from the overflow tube. Youll end up with a big air bubble in the engine and an overflowing overflow tank. Given that ill be running an unpressurized recovery tank i believe the two caps shown below are the correct type. The radiator cap is not the highest point in the cooling system. The cap will not allow air or coolant to expand out of the radiator until the design pressure is met 7 or 13 lbs depending on model at which point you may have an overheating condition.
Test or replace the coolant cap when servicing the cooling system. Made from premium materials. But there are many conflicting opinions on this. A non vented cap will not allow pressure to enter the system. The swiv el design allows the cap to be installed and removed much easier than conventional radiator caps.