On Tue, 14 Aug 2018 19:51:10 -0400, Wade Garrett wrote:

Seems to me though if you blow the air out of the line through the
loosened connection then turn the gas off prior to re-tightening the
connection, air will get back in the line. No?

No. Like "bleeding" anything else, you're dealing with a positive pressure against
atmospheric pressure. The atmospheric can't overcome the positive.
But I've never had to "bleed" air out of natural gas lines. The appliance does that.
A 20 year old dryer probably has a pilot light. That's connected at the gas valve, so
lighting the pilot serves to bleed out the gas line when it's been disconnected.
With more modern dryers, a few repeated lighting cycles will do the same.