I have an '88 westy that has the following symptoms:

After about 3 hours freeway driving (~4200 rpm) The red led will flash and
the temp gauge climb to the top of the scale. It appears that the coolant
tank fills with air - and coolant gets pushed out to the overflow tank -
which fills to above the cap. At some point the pressure starts spraying
coolant out the back of the van.  (Good to keep tailgaters at bay)

After inspecting the engine I found no leaks, but did find a leaky rear
heater core. I replaced the core and thought all was well.

On a test run - sure enough, 3 hours at (~4200 rpm) and the same thing
happens. This time I stopped and kept the motor idling to see if I could
discover anything else. I saw air bubbling into the expansion tank. In
trying to see where the air was coming from, I followed the plastic loop and
noticed that if I pinched closed the small hose from the right cylinder bank
to the plastic loop - the bubbling stopped.

Are there places that the cooling system will leak that are not easily
found?
Also, how do you check to see if the heads are leaking?

Thanks,
Todd '88 Westy