Gurgles

My sister complains that when her kitchen sink is full and she pulls the plug it just glugs and empties very slowly. If it isn't completely full though it empties fine.
I'm thinking that probably she needs a trap with an integral air admittance device, can anyone confirm this or offer any other advice? Thanks.

aav

That is the kind of problem that an AAV will either not cure or make worse. She has a drain problem, and maybe two. The first one is that the drain must be plugged or at least partially plugged, and she may, or may not, have a vent. If she does she does not need an AAV, and if she doesn't have one an AAV will not improve her situation.

No blockage in the pipe apparently. As the problem only seems to occur when the sink is full I think the trouble must be something to do with a full bore discharge and so is air related. Maybe a bigger drainage pipe or reducing the fall of the pipe would sort it out? Why would an air admittance valve exasperate things then?

It's VERY possible and I'd guess even probable that she has a clog. The drain cant handle the amount of water it is getting when the sink is full. The pipe fills up from the point of the clog and slowly leaks by the clog. When the sink is full the pipe fills up completely, fills the trap arm, trap and tailpiece (and/or continuous waste) showing evidence of a clog....the "full sink of water has nowhere to go. When the sink isnt completely full there's enough space in the pipe before the clog to accomidate the amount of water discharged.