I wonder if I have a defective cap then? Been using the non vented style (picture on the right) and never had any pressure build up issues. It might be because the seal is worn or missing and infact acted as a vented cap. I bought the new vented design (pic on he left) and now have issues with tank pressure building up.
If I blow into the vented cap, air will not flow, but if suction is applied, air will flow. This indicates pressure would build in the tank but not allow a vacuum to build.
I've got the vapor return hooked up now and still had pressure build up in the tank. I might just use the old "non-vented" cap for now.