Original post by: blackwood ,
'''It could be one of three things, possibly the governor but not usually the cause but they're fairly cheap and easy to swap out to check and it's always good to keep a spare anyway. Could be the purge valve in the bottom of the air dryer. You might crawl under it while it's running and see if there is any air leaking out the bottom of the dryer, also fairly cheap and easy to replace. Most likely culprit if you have a Bendix air dryer it will have two air lines coming off the air governor probably #4 stratoflex or braided air lines, one will go from the governor to the air tank and the other will go from the governor to the air dryer. As the other poster said spray these lines down with soapy water or 409 and look to see if they start foaming up, if they do replace these lines. They are bad to dry crack under the fabric of the line due to excessive heat around the engine and you can't hear the air leak because the fabric dampens the sound.'''''italic text''