Slow draining kitchen sink - very frustrating!

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Various threads about this, but still puzzled about my problem.

Kitchen sink with dishwasher and overflow connected into waste above trap. Trap leads to vertical downpipe running out beneath floor.

The problem is that if water is run or dishwasher drains, it backs into the sink. If I release the pipework after the trap, air is forced out and the water drains rapidly away. It will drain if left, but only very slowly.

If I disconnect all the waste pipework, I can direct water from a garden hose down the vertical pipe at full pressure, with no backing up at all.

I've tried fitting an air admittance valve onto a 'T' at the top of the pipework, but with no effect. The air wants to come out not in.

All this in 40mm plastic pipe. I've tried swapping from the U bend to a bottle trap, but no difference.

Probably just coincidence?, but the problem appeared when we swapped to a new dishwasher - the waste is plumbed in above the trap exactly as the old one.

Does anybody have any suggestions? - I can only think it's some weird external drain problem, but kitchen backs straight onto garden, and I can't see where there's an appropriate access point.
 
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Hi I think you will find that you have a waste pipe that has sagged, and
holding water, thus creating an air lock between sagged section of pipe
and the trap. so the air cant get out and water cant get in to waste pipe,
 
Thanks for the response - not sure which bit of waste could be the problem. There's a short (4") horizontal section from the trap to the downpipe. Unfortunately, no other part of the system appears accessible.
 
can you remove the plinth and look under sink unit you may need to remove back pannel
 
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Hi - the downpipe (which does have a dogleg in it as it goes down) just dissappears into the concrete floor. I've had the plinth off.

Remember that I can pump as much water from a hose as I can into this pipe, so there doesn't seem to be a blockage. Obviously I can't tell if much air is being displaced back up the pipe as I do this.
 
does it go into a gulley outside or a manhole which could be partially blocked and holding water
 

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