Maybe you don't even have to rip the guts out of the cap, because that
cap has two pressure valves in it, one for each direction -
a low pressure valve (a few PSI) in the direction from the reserve
tank towards the expansion tank, and 1 bar (~14psi) in the reverse
direction.

First you'll need to test the low pressure valve (because that's the
direction to pressurize the system). Simply blow into the hose union
and see that it's easy to do so (just a few PSI) .

Now for pressurizing the system: Get any manual air pump with a gauge
(could be even a cheap foot-pump, just not one of those small 12V
compressors that can go as high as 250psi...), improvise a connection
to the hose union on the cap and pump into it (i.e. pressurizing the
system). Since there's a low pressure valve in that direction, system
pressure will be very close to what you see on your gauge. Measuring
the coolant system pressure isn't so easy because the valve in that
direction is about 1 Bar (14psi), but can be done indirectly.
To release the pressure, you'll have to release the cap.

BTW, I haven't tested the above myself yet, will try when the time permits.
Good luck!

On 9/21/07, Kenneth Wilford <kenwilfy@comcast.net> wrote:
> Make your own out of an old pressure cap.  You can rip the guts out of an old cap so that you can just pump it up without the valving system causing a problem.  Voila! instant adapter.
>
> Hope this helps,
> Ken Wilford
> John 3:16
> www.vanagain.com
>
>
> -------------- Original message --------------
> From: Zoltan Kuthy <zolo@FOXINTERNET.NET>
>
> > Hi List,
> > I have a pump to pump up a normal radiator but I guess, I would need an adaptor
> > to it to be able to pump up the Vanagon system. Anyone knows of one and where
> > to get it?
> > Zoltan
>