Mike if there isn't any combustion gasses in the coolant then where is the
pressure coming from?  Have you checked for a stuck thermostat?

If the head gasket was leaking (the metal one that would leak combustion
gases into the coolant), then you would defineatly get combustion gases in
the coolant itself.  What you describes sounds like this but if a sniffer
isn't picking anything up then it must not be a cylinder leak.  It could be a
stuck thermostat, or possible miss hookup of the coolant hoses that go to the
radiator in the front of the van.
I hope this helps you but I wouldn't pull those heads just yet.
Ken Wilford
Van-Again
John 3:16