I think people are making too much out of purging a tank.  When it runs out of propane as they often do, it is filled with air.  When the bleeder is opened while filling the air is expelled out the bleeder.  When liquid propane starts coming out the bleeder, it is full.

--- On Wed, 10/3/12, Mike S <mikes@FLATSURFACE.COM> wrote:

> From: Mike S <mikes@FLATSURFACE.COM>
> Subject: Re: Propane Tank question
> To: vanagon@GERRY.VANAGON.COM
> Date: Wednesday, October 3, 2012, 12:15 PM
> On 10/3/2012 11:32 AM, Stuart
> MacMillan wrote:
> > Problems like this are maddening!� Bad auto stop
> valve or bleed valve that
> > doesn't vent are the only things that can prevent
> filling.
> 
> It's more likely that the tank simply needs to be purged of
> air, as
> already mentioned.
> 
> When the tank has nothing but propane in it, it can be
> easily filled.
> Pumping liquid propane in increases the pressure, and the
> existing
> propane gas in the tank turns to liquid, making room. For a
> tank at 80F,
> that pressure is about 130 psi.
> 
> If you try to fill a tank which has air in it, the air
> simply
> compresses, and pressure goes up. New tanks are often
> shipped with
> compressed air in them, so you can't push much propane in
> before the
> pressure gets too great to be able to pump more.
> 
> Hence, the need to purge the air from the tank before first
> use.
>