Finally found the article that made me check the check valve. This goes into some additional stuff and may be something you want to do, but just futzing with the valve and cleaning it fixed the problem for me. Worth doing...so much nicer to drive without the fumes.
This is from the compilation of Owner's Technical Notes Series from the AROC. I hope I have not stepped on any toes by posting this. It is a very handy publication with lots of fine tips and guidance.
Good luck, Jonathan