There's a section on bellows maintenance in MORE POWER TO YOUR ELBOW. The companion volume, THE WIND IN THE BELLOWS, is also a must-have. They are published by the LBPS and available for order elsewhere on this site.
It could be the clack valve. Make sure it's working properly before putting stuff inside the bellows. Also, check the bellows leather. I've seen bellows that were made of inferior leather or leather that was rotten from oil that no amount of beeswax and neatsfoot oil would make air tight. YMMV, but in my experience if you need to season your bellows then you probably need a new one. A good bellows with two layers of leather, sown, with a clack valve that closes immediately, is expensive, but well worth the price. It's not worth it to have an inferior. Do yourself a favor and get it out of your life.
