I forget where the vent is on that tank,I'll have to look at my diag's..
Change that cap out for the correct MB cap.
Drop me an email and I'll send you the diagram of the vent/carbon system, you should be able to disconnect it @ the back of the bottom of the tank.
blueridgemb@gmail.com in case you don't have it handy.
Good call on the cap idea..it's been a while since I've come across that
jonathan