Depending on your state, whether or not you have to have the tailpipe "Sniffed", and how you have it registered, Removing "All that smog" equipment isn't an option.
That valve may be a One way check valve for vacuum. You can rob one off any later model import even with EFI. May not be the correct colors,but will do the job. Check to see if it is by blowing through it one way, and the other way you shouldn't be able to.
May also find you will need to replace all of the billion vacuum lines running everywhere on that thing.
Those year cars were notorious for these type problems. Half electronic carb BS. Good luck.
My point was not to do away with the smog but disconnect all of it, then work your way backwards to find the trouble area. It's a technique used in trouble shooting computers, just a thought