T4 water tank filler pipe⁹ vomiting

[richardvwt4]

When our t4 water tank is nearly full it start to vomit. It emits water up and refuses to take in more water even though there is clearly a finger length left to be filled.
There is no special design in it's swing pipe, straightforward, so what is happening? There are tw0 big washers and nothing more so why does it suddenly refuse to fill completely?

 

[sidepod]

Taking a wild guess, I’d say an airlock.
I assume from your description you have the early type of tank, in the boot with the swivel neck?
If it’s full apart from the neck then that begs the question, how full do you need it?
You could try filling it till it “vomits” , put the cap on, drive round the block and try again.