Being that Volvo uses a "fixed orifice" system as opposed to a "variable flow" system, there is no "retractable pintle". Isn't it this pintle which effectively closes off any return pressure in case of a backfire? The open nature(lacking a hard mechanical stopping mechanism) would seem ill suited to anything but regulation of crank case pressure. I am no engineer but that plastic screen would seem unlikely to buffer the back pressure significantly compared to a pintle being forced back into its seat. Just thinking aloud here...

"When the engine is off or it backfires, spring tension closes the valve completely preventing the release of blowby into the intake manifold. The valve closes during a backfire to prevent the flame from traveling into the crankcase where it could ignite the enclosed fuel vapors"

Source: http://www.autoshop101.com/forms/h63.pdf