Re my cooling system problems:

Brought the van to operating temp, let it run for awhile, looked
everywhere for leaks, saw none. Water level in expansion tank rose a
bit, overflowed properly into fill tank. Upon cooling down, water level
in fill tank receded to just above 'high' line. So the pressure cap is
doing it's job, just not clear if it's doing it at the right
temperatures. The pressure cap is the 'blue' type, but I don't know how
old it is. I'm thinking of replacing just as preventative maintenance,
though.

Re luggage rack seal:

I ordered the BusDepot"bulb" type seal after experiencing alot of panel
drumming noise with their "bulbless" seal. Installed it Saturday, took
the van for a highway drive yesterday, with lots of wind (summer in San
Francisco!) I have to report that the panel drumming is gone, confirming
my supposition that air was entering the area between the rack and roof,
causing the racket. If my anecdotal experience can be relied upon, I
would recommend the "bulb" type seal for both the poptop and luggage
rack. These are really nice seals!

I also installed a new, replacement skylight seal from BusDepot, and the
design appears to be an improvement over the stock seal. It's basically
a 'bulb' seal similar to the poptop and luggage rack seal designs. Plus,
you don't have to remove or even unfasten the skylight; the seal can be
installed with everything in place.

Thanks BusDepot! (and others who have advised me on my cooling system!)

Dan