Yes, you have that right. It is a special fitting that Volvo used.

That fitting allows them to pull a vacuum and then fill the system in one swoop on the assembly line.

I surmise that it is the large port accessibility and that is the problem. The surface area of the seat diameter and the seal inside the body lends itself not to seal under low-pressure side conditions. In short, it was a mistake.

I fixed mine when I had the system discharged to a zero pressure condition. I took a metal slug out of a used electrical receptacle box. I used a sharpen oilstone to make the slug flat and the valve top smoother.

I cleaned the two surfaces with denatured alcohol. I then applied super glue to the mating surface to form a capping plug. No more leakage plus there is still the black cap for back up.

It valve still could be used if it ever goes back to the assemble line. Yeah right!

Phil