I believe Volvo calls it an accumulator. Some say it is a pulsation dampener, to smooth out the flow pressure to the fuel rail. One of my other make of vehicles has one on the rail and there isn't any vacuum conncted to it. Members on other Volvo forums claim that the accumulator becomes leaky and so it cannot hold the fuel in the lines and hence the long cranking attempts. For me , even after a start , I believe I was not getting the full pump pressure at the rails and this cause the stuttering and backfiring when I tried accelerating. I took mine apart and honestly , it looks like just a two chambered device with no moving parts inside. So where it leaks , I think, is at the attachment points. The device merely pushes on , at both ends , to the metal fuel line. The material there is rubbery and perhaps over the years it has lost its sealing ability.