We're having trouble stopping our kitchen sink drain from backing up. The sink now backs up nearly instantly (after a few seconds), previously it took a bit longer to backup.
We've tried on multiple times with different methods to fix it ourselves including with a plunger, alkaline drain un-blocker as well as caustic soda.
At one point, the bathroom sink (close to kitchen) also backed up. From that we deduced where a blockage might be and unscrewed the pipe and cleared out a lot of white sediment that had built up along the bottom side of the pipe. This "solved" the problem for around a day, but then the problem returned.
After this, we researched another method that involved pushing a 3m spring/screw down the pipe in order to try and pull up a blockage. We did this from the sink end and from the other end of a near 6m inaccessible stretch of pipe that ends near a junction of piping (see diagram below). Nothing was recovered from this, it seems as if there may be no physical blockage.
We've all but run out of ideas at this point, but some research on the internet suggests that the problem might be related to air/suction, it seems to fit with (1) the blockage occuring nearly instantly, (2) not finding a physical blockage & (3) we've unscrewed the pipes at the sink and a junction enough times that we could easily have introduced a problem without realising it, not being plumbers. I've also confirmed that there is no water backed up near where the pipes all join up before going underground.
We have a dishwasher that also feeds into the drain straight after the kitchen sink. This has some sort of non-return valve which we've also unscrewed a couple of times and at one point it leaked causing us to screw it up very tightly, which could have introduced a problem for all we know. We're not sure how it works, or whether we should have screwed it up tightly, but the problem appears to remain even when unscrewed and holding a hand over the opening.
I've included some photos and a diagram, on the off chance that it helps, it's a problem that's been with us for a while. The diagram shows one of the difficulties with diagnosing the problem, in that a near 6m stretch of pipe after the sink is not accessible.
If no physical blockage is present, what would be causing the sink to backup? Is there some kind of air-trap or pressure problem that could be the cause?
Any general advice for how to investigate/fix is welcome, particularly as the covid situation makes this a bad/impossible time to get a professional to investigate.
Update I doubt it matters, but just in case, we're in the UK. Also, there are no visible drain vents anywhere along the system we're discussing. There is however a central heating system feeding into the same drain and the seal is not air tight, with two 1.5cm pipes feeding into the drain.
Also, all of this plumbing is on the groundfloor.
Under kitchen sink
Specifics of what solved it
Obviously all of the advice on here was sound, but I wanted to highlight some of the specifics that helped solve this, in case it helps someone else.
The problem turned out to be a simple blockage, a build up of sludge, but our diagnostics turned out to be flawed. In the end on advice from this question, we bought a 6m snake - https://www.amazon.co.uk/gp/product/B000O55KJC/ref=ppx_yo_dt_b_asin_title_o01_s01?ie=UTF8&psc=1
The above worked well, and far better than our 3m snake without the blue attachment.
The specific technique that helped was to time how long it took to back up. From the volume of the pipe and from how much water your tap flows per second you can then work out where the blockage must start. From that we could tell the problem wasn't the piping around the sink, it was the inaccessible part.
We then proceeded to snake it, two techniques helped, turning the coil when a blockage was struck, but also sliding it backwards and forwards. We undid the bathroom sink junction at the opposite end to see if the snake was through and when we came out the other side. We then put the garden hose pipe down the sink and turned it on, it still backed up, so we continued snaking and looking for the blockage. Eventually a lot of sludge came out and we had an idea we had made progress. It's not been long, but the sink now drains. And if it backs up again we have a fair idea that the same procedure will help.
Thanks again for all that helped us fix this!
take off all the kitchen piping