Coolant not Returning to Radiator

[wortus]

I had a problem with my V6 coolant not returning to the radiator once it cooled and the level would always drop. I replaced the cap a few months ago and it fixed it. It's started doing it again, ther eis no air in the top of the motor. The level drops but is still visable in the radiator so it's not that low and takes about 50 to 100ml max.
How much lower than the bottom of the cap should the water level normally sit when cold?
The overflow is just filling up when I top the radiator up so there is no leak and the hose from the radiator to the overflow is fine, no splits and it's not loose. Any ideas on what's going on here or is this normal?

 

[db_notso]

check the hose isnt too short as in it sits in the water in the bottom of the overflow
check ENTIRE hose for cracks etc

else be aware that you can never eliminate ALL air from a motors cooling system

no matter how many times you bleed it there will still be a bit there

HOWEVER you should never run out of coolant in you radiator when the resevoir is full
UNLESS there is a coolant leak big enough for air to be sucked in instead of water being sucked in from resevoir

the system works due to a difference in pressure between atmosphere and the sealed cooling system
any connection between the air and coolant other than cap can cuase this fault and is VERY VERY UNCOMMON
worst case ive seen the car would only leak when cold due to a hose clamp not being tight enough on a steel water outlet