RW> ATF mixed with coolant (coagulating) probably didn't have enough time to
RW> circulate through the core, thus building up a pressure head at the
RW> weakest point in the system...upper hose.

That is exactly what happened.  

RW>  MH> The overflow was also overflowed.  :)
RW> 
RW> The only pressure relief point. 8^)

What is strange, is the overflow's cap didn't pop off.  The entire overflow
tank is plastic, too.  The thick upper hose ripped in half, however.

After I opened the overflow's cap, it was completely full to the top with
that goop.  Normally, the antifreeze is at least 5 inches below the top of
the overflow.  

- Mark

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