It should. Unless there is a pin-hole in either the rubber hose, or the aluminum tube that runs under the battery. Either of those scenarios would allow air into the system.

 

I kinda thought it should be full to the rim also but blowing into the hose to the overflow confirmed that it wasn't clogged and shouldn't the pressure test that I done on the system revealed any leak to the overflow tank ?

 

It depends on how the pressure test was done. If it was a standard test, it didn't check the overflow tube or catch tank for leaks.

The coolant catch tank has a hose that leads to the radiator, and another hose that expels excess coolant to the ground.

A standard cooling system pressure test that connects to the radiator fill won't pressurize the catch tank, as it won't hold pressure and isn't designed to be pressurized.

If you have a pinhole in either the rubber hose, or the aluminum tube that runs beneath the battery, you could easily push coolant to the catch tank when the engine is hot, but it will suck air and not coolant back into the radiator as the engine cools.