Slow draining Wash basin

[John and Jo]

Just to confirm, are the "hand sink" and "bath sink" both referring to the wash basin in the toilet?

Yes I was sorry I didnt make it that clear.

My Kitchen is larger also, and that might make some difference but my moneys on the trapped air theory.

My pipework is 28mm rigid pipe. Some of the fallaway is not great but the kitchen sink is fine and it all goes down the same pipe eventually so I'm ruling out blockages.

I'm also thinking of a breather or even fitting in an overflow but not too keen on drilling my sink.

 

[rosalan]

My kitchen sink drain is excellent unless food particles begin building up.
It is the vanity basin, situated in the centre of my van, that is slow. Oddly one of the two shower drains can be slow to drain, depending on how level the ground is. This could be a clue.