Back in the days before air dryers, truckers that used their rigs everyday would purge the tanks every day. Open the valve until all visible water was gone, then close them. You weren't letting out enough air to drain the tank and the next time you started the engine it built right back up, so no problem. Once a month, or every two weeks in freezing weather, at the end of the day you'd open the petcock and let everything drain out to zero. Sometimes we'd even try to park so the tanks had the petcocks downhill so they'd drain everything.
And every so often we'd pour gas tank anti-freeze into the trailer gladhands to help absorb and expel the moisture that was in there. Occasionally someone would get enough moisture at just the right time to freeze up the air compressor governor. And if it froze shut you'd get no air into your tanks and would slowly run out until your spring brakes came on and you were stuck sitting wherever you were. Then, sitting for 20 minutes would thaw out the governor since the engine was hot, and everything would work fine until just the right conditions came by again. Or until you replaced the governor and blew out the system. Ah the good old days before everything was automatic and foolproof.
