""""First the reservoir failed, then the radiator and now the H piece on the
radiator.""""

Well i still don't understand the whole story? if you want us to help you
can you start from the beginning, what year is you van VWNut?

What is the H part?
How is the engine temp?
What was change (parts) lately
Did you manualy check the pump for loose or wear?
Did you check the thermostat in  boilling water?

Reservoir?? Expansion tank!!!

Ok, i think i know what you mean now buy "overpressure", everything is
blowing up!

Head crack??? have/do a pressure test:
http://www.benplace.com/pressure_test.htm

I do hope the expansion cap is new!

Let us know, am sure that many guys can help!!!

Ben