Strange Blocked Sink

[Hungry Horace]

My kitchen sink is blocked, if I run the tap for 30 seconds the water starts backing up. The fittings under the sink are the usual plastic trap with a dishwasher connected and the overflow goes to a secondary smaller sink (where we keep washing up brush etc). After the u-bend it goes straight into the wall and into the drain. The kitchen is located on the first floor and there are other rooms on this floor with sinks, washing machine etc but these are all fine.
I have disconnected all the fittings and cleaned them but they were not blocked, I then ran a hose from the tap straight into the hole in the wall and ran the water for 5 minutes and it didnt back up, reconnected everything ran the tap for 30 seconds and it backed up. Afterwards the sink was making a glugging sound for quite a while so I ran the tap briefly and the noise stopped but it is still backing up.
As you can guess I am at a total loss as to what the cause is, any ideas or advice would be greatly appreciated.

Many thanks

 

[stevetheplumber]

obviously bloked further down

 

[Hungry Horace]

Thanks for the reply. Your statement that its obviously blocked further down seems obvious but how come if I remove the trap and put a hose straight into the waste hole in the wall and run the water it doesn't back up. This tells me that there isn't a blockage further down, hence why I posted on here as it doesn't make sense to me.
I should say the sink does drain just very very slowly.

 

[kay-jay]

have you left the plug in?:icon6:

 

[kay-jay]

only kidding!

has it always drained slow? or was it once ok? is the pipework ok falls going the right way etc?

i had a similar problem once and it was a disc of congealed fat, i undid and ran water through as you did reassembled and tested only to find still not draining. it was moving in pipe and becoming lodged again but sometimes letting water past. so don't just run water through actually shove something through the trap and pipes

 

[Paulus]

Alternatively try a power flusher . Available from B&Q for about £12. It sends a jet of compressed air through the pipes from the waste. Its never failed me yet.

 

[Hungry Horace]

The sink did drain ok it just started happening a week or so ago, so initially I thought it was blocked. All the falls are ok as I didnt touch them until the problem occured.
Also I totally cleaned the trap so there is definately nothing blocking it. I have also put a snake spring in the pipework to a length of 10 metres and didnt come against a blockage only a few stiff areas which I guess were bends in the pipework.