As I was re-installing the fresh (drinking) water reservoir in my westy, I
noticed the that clear plastic hose, that vents the top of the reservoir to the
plastic filler hatch on the side of the van, was about 1.5 inches longer than
need be and had a kink in the hose cutting off the air trying to escape from
the reservoir as it fills up with water from the outside fill hatch.  I cut the 1.5
inches off and the fit is much straighter now with no kinks to obstruct the air.

I don't know how you'd check yours (without taking the van apart), but you
could try timing the fill as is, then re do it, with the top OFF the reservoir (or
the air hose taken off the reservoir- see below) - time it again - if there is
appreciable difference, then you might have a kinked air hose.  Another way
to test it, would be to take off the hose from the top on the reservoir and try
and blow through it. pull on it - a bit - and blow again.  In taking the hose off,
you will ruin the clamp, so be sure to have another all stainless steel screw
clamp to replace it.

While we're at it, the hoses in my 91 were a bit dirty and grungy INSIDE, I
will be changing all of the hoses with new clear plastic hoses.  Malcolm S