Sink draining very slowly yet U-bend is clear, etc

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My sink's U-bend gets clogged up with gunk every few months so I need to dismantle the plastic U-bend and clean it out. I just did this again yet still the sink is still draining very slowly. After the U-bend is a very straight piece of pipe that leads to the outside (where it drains into my (private) drains (I have a septic tank and soakaway - all of that is perfectly clear)).

The obvious answer to this conundrum is that the straight length of pipe after the U-bend is also partly blocked but it's in such a hard to access place that it's hard to tell.

However, as the U-bend under the sink catches most of the gunk I can't imagine that much would get into the straight piece of pipe, let alone accumulate there.

Is there any other possible explanation for the slow sink drainage?

If not then it's time for the plumber.
 
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Anywhere on the waste pipe run may be restricted . given the fact that you have to clear the trap every few months the pipes a distinct possibility
 
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Thanks. The waste pipe segments are glued together and in a hard to reach place, I guess I could have a go at removing it to inspect it properly ..... or clall the plumber.

What type of glue is normally used on plastic waste pipe joins? (whatever is on mine appears to be yellow). I guess it won't just pull apart with force?
 
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Remove the trap and see if you can snake the pipe through with a flexible drain pipe cleaner from Toolstation, etc. If not possible to do through trap, cut pipe, snake, and rejoin with a Fernco coupling or solvent weld coupling.
 
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Sounds like solvent welded fittings which are not designed to come apart. There are tools available to wind down waste pipes to clear blockages ,snake like.
 
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Problem is a snake is generally a much smaller diameter than the pipe so can lead to a false account. Try buckets of very hot soapy water.
 
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Thanks for all of the suggestion, I'll invest in a metal snake tool and give that a try first, then take it from there.
 
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How long is your very straight piece of pipe and how much fall does it have on it? (had a similar problem myself years ago, got fed up of pulling u-bend to bits every 3 months so redid the waste run to replace the slightly (1:100 or worse) falling 1500mm of pipe with a significantly falling (1:50) 1600mm. Problem gone away.....
 
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