Bathroom sink very slow drain mystery

[L.F]

The bathroom was refitted 10 years ago. Everything was fine until a couple of years ago. The basin is very slow draining and you can see the water coming up from the P trap and then filling up the basin. I've tried drain unblockers etc. and also put a hose with a snug fitting nozzle into the downpipe. To my surprise the water flows freely at full hose pressure (which suggests the drain isn't blocked), yet, if I lift it out of the downpipe it fills up the basin. The P trap is clean and doesn't seem to have anything wrong with it. Should I try another P trap, please, or does anyone have any other ideas?

 

[Deleted member 120897]

Check the overflow isn’t blocked, and therecould be a blockage further down the waste pipe.

 

[Ben-gee]

Remove the trap and thoroughly clean the waste fitting and grid/strainer from above and below.
It never ceases to amaze me how what looks like a bit of soap and a few hairs can cause this problem. I find cotton buds and a small screwdriver good for this, if you have an o-ring picking tool even better.