Thetford C402 flush filling issue

[hadders]

Hi, we've had a few motorhomes all with the same loo cassette type. Our latest one has a C402 Thetford which has a separate filler neck you pull out for the flush water and level tube.
The dealer initially primed this for us but just tried to refill after using this up. Seems it doesn't want to drain down from the filler neck and after reading the basic instructions, I used the supplied funnel which helps push some fluid into the tube but it still sits in the filler neck - is this normal?
I undid the tube and allowed fluid to pour out which happens though the tube doesn't show full and the neck still is!!! Thought it was an airlock but any suggestions? Thanks in advance Andy

 

[TheBig1]

It's a design flaw that the top filling neck retains water. so thetford fitted a different filler neck with a little nut on the end to drain the filler from. when filling, just keep filling the filler neck and it overflows into the holding tank

 

[hadders]

It's a design flaw that the top filling neck retains water. so thetford fitted a different filler neck with a little nut on the end to drain the filler from. when filling, just keep filling the filler neck and it overflows into the holding tank

Not noticed the nut, but thought a drive would have budged the fluid in the neck but until I'd used the funnel attached to the neck, didn't get any to flush with.