>>After reading about all of these head problems I'd like to understand what 
>actually 

This is a corrosion problem, and its rate is thus determined by the 
environment. The type of antifreeze, amount of air in the system, 
temperature, quality of alloys used in the heads etc. Thus YMMV. 
        I still have to compile the results from a data request I made a 
while back. NOTE: only 23 respondents out of 130+ Vanagon posters!!! It 
appears that leaks appear any where from 50-60K miles to ?? (never?). 
Repairs last 2-3+ years. There appear to be three types of leaks, water out 
to ground via head gaskets, combustion gasses into system via cyl. barrel 
gaskets, air into system via inhalation on cooldown. 
        Concurrent problems are everyday leaks at any one of many hose 
connections, rad/heater cores etc. Faulty/dying water pumps figure into this 
often. Net effect is air in the coolant system, which MUST be bled out 
properly at front rad. bolt. Rumors of better heads and pumps from '87 on, 
my '87 leaked and leaks still though. Just look for whitish encrusted areas 
on your heads.
        Most of these leaks would mean nothing on a regular car, just 
ocassional painless topping up of the rad via the cap. Due to the position 
of the rear reservoir/cap, its low height relative to the water pump, and 
the high front rad this is a major problem on the Vanagons. I haven't heard 
anyone mention water in oil, which leads to brown foamy oil, yet.    tim s.