very odd problem !!

I have a very slow draining sink in the bathroom.
I have removed the p trap and cleaned it out.
I have cleaned the other pipes and can pour water down them successfully.
I can pour water throught the p trap when it is not connected.

HOWEVER - when I reconnect it all , after only 1 cupfull of water down the sink it starts backing up .
How can this be ??
The p trap works fine when i disconnect it and run water through it and we have NO blockage

Really Odd - HELP !!

Thanks
Ian

Slow drain

I've seen older homes that did not have a vent for the bathroom basin. That's what it sounds like to me. Check to see if there is a vent or if the vent is blocked. If there is no vent at all, you might look at a Studor vent for the basin.

Remove the pop-up and inspect inside the drain.. There may be an obstruction in the overflow holes in the tailpiece.

Also, make sure your pop-up is adjusted so that it goes up all the way.

If as little as one cup of water backs up I agree that the problem is right there in the pop up drain assembly. An amazing amount of "gook" ( soap mixed with decaying hair) can build up there.