Hepo valve. Possible vacuum lock?- fanny trap puzzled

I have a Hepo valve, http://www.hepvo.com/ AKA fanny trap valve (40mm, 1.5"). Under a basic on a kitchen island. It was functioning fine for 3 years unto recently the sink appeared to be blocked. Have cleaned out the tubes (think I have done this sufficiently) using a brushed rod, I even disassembled the overflow on the blanco sink and clean that out too. Clean the outside drain that meets the main kitchen sink, and got my hands in there to clean it out of stone and other debris. I found during this cleaning that pulling a black inner from main drain released some vacuum lock and clear the drain. A few days later, the sink blocked again. I proceed to remove the hepo valve and test the remainder of the drain by pouring water in the last run of the pipe. From this process of elimination it appeared that the hepo valve would be at fault (simple design that it is).

Today I replaced the valve with a brand new (not cheap) appears to be exact match as I took the old one in to the plumb shop in Baldock - Didn't work - "Same fault".

Now I'm lost for ideas.

I suspect that there is a vacuum lock of some sort occurring, as I remember in the past I had slightly loosened the valve and this did clear the water, but resulted in drips.

I'm assuming the ribs on the valve doesn't matter in this vertical installation.

I'm not sure if the outside drain needs the black inner checking or I am missing something on the installation.

Any ideas would be greatly appreciated.

it is not approved for use on residential/commercial plumbing only on an RV