Here's a Political Action Committee (PAC) site that explains some of the OPD
history.  Thanks to Mark for pointing out that my "It seems" opinion was
perhaps unjustified.

http://www.npga.org/i4a/pages/index.cfm?pageid=531

When reading the article, remember that these folks are the lobbying
organization who represent the Propane Gas Industry.

So if you DONT open the bleed valve on an "empty" tank, how much liquid
propane will flow into the tank from the source and what determines this
amount?  I personally watch the work done by the purveyor in filling the
tanks to ensure that the filling vent is open and that he stops filling when
liquid phase propane exits the vent.  I've caught a few who had a little
difficulty in getting the autostop to accept a fill, opening the vent
usually fixes this.  On occasion, the release "button" must be reset to get
the tank filled.

Pensioner