Kitchen sink not draining, maybe airlocked?

[BorisUK2000]

I am hoping someone can help ... our kitchen sink stopped draining, it looks like an airlock somewhere. This is what I tried:

1. Used chemical drain unblocked and plunger
2. Took everything apart and checked all pipes
3. Bought endoscope and inspected all pipes all the way to 110 soil pipe - all clean
4. Replaced bottle trap (McAlpine C10V Anti-Vac). Also tried McAlpine S Trap
5. When the sink is full and not draining, I can push the AAV up with my finger, then the valve starts sucking air in and the sink starts draining. This is why I suspect the issue is with the airlock somewhere

This is our current plumbing, many thanks:

 

[Deleted member 120897]

Is this the only drain affected? Is the soil pipe vented or air admittance valve (aav)?

 

[BorisUK2000]

Is this the only drain affected? Is the soil pipe vented or air admittance valve (aav)?

Yes, this is the only drain affected. Everything else in the house drains nicely. I believe the soil pipe is vented into the loft (I know the others in the house are, but not sure about this one).

I don't think the waste pipe is blocked. I looked inside using endoscope. It is a large 110mm pipe which could take a lot of water. The sink stops draining as soon as the waste fills up, for example if I cover it by hand, fill up 5L of water into the sink and then remove my hand.

I am thinking about installing another AAV, just under the worktop, as per this picture. Do you think that would help?

 

[BorisUK2000]

Is this the only drain affected? Is the soil pipe vented or air admittance valve (aav)?

I forgot to mention, I removed the P trap and poured a bucket of water into the 40mm waste pipe and it was running nicely.

With the P trap connected, if I pour the same amount of water into the sink quickly, it blocks immediately. If I pour the same amount of water in slowly, it runs, as long as the waste is not fully covered in water.

 

[Deleted member 120897]

Is there an overflow connected to the sink waste?

 

[Ben-gee]

You are saying p-trap but what you actually have connected is a bottle trap, these are often really bad on kitchen sinks.
The first thing I would do is change your trap to a p-trap.

 

[Ben-gee]

I would also cut the waste lowdown to remove the two 90 bends and come straight up with just one bend at the top, make sure it’s a swept bend not a knuckle one.

 

[BorisUK2000]

Is there an overflow connected to the sink waste?

Yes, see this picture. A few days ago I found that the overflow pipe was blocked with black muck. I cleared it and thought that made if better, but will check again.

 

[BorisUK2000]

You are saying p-trap but what you actually have connected is a bottle trap, these are often really bad on kitchen sinks.
The first thing I would do is change your trap to a p-trap.

Sorry, I meant bottle trap. I also tried McAlpine Sink S Trap 40mm but that had the same problem. I changed it back to the McAlpine Anti-syphon Bottle Trap as the Anti-Syphon Valve did a good job stopping gurgling caused by the dish washer.

 

[BorisUK2000]

I would also cut the waste lowdown to remove the two 90 bends and come straight up with just one bend at the top, make sure it’s a swept bend not a knuckle one.

Thanks, I'll try that