This .pdf of Group 4 should help. On my 1982 I don't have the canister (#4).
Check valve is (#9) located under rear deck lid cover behind and above rear seats.
Problems usually occur when oily residue accumulates in areas 3, 4, and 6; occasionally blocked check valve.
Greg Gordon has an outstanding a tech bulletin on this:
Here are 1987 Tech notes from the late great Fred Di Matteo (Velocissima-AONE)
And for the 1983 and before models; EMM.pdf attached, section D-21 from Group 4, probably best shows the check valve in question #3 (notice no charcoal canaster; in 1983 and before from oil separator to firewall then hardline (#2 - on vehicle interior) to rear of alfetta where check valve placed in line prior to vent line and expansion tank)