Interesting as well I notice the bottom hose on the radiator is
leaking a little and could pressure be the issue with no flow?
My hypothesis would be that as the engine cools, it sucks air in through the
leak hole rather than sucking the coolant back from the overflow tank.
When next you start the engine, the air trapped in the engine heats up quicker
than the coolant, and expands quickly, forcing the coolant out the easiest way
- through the radiator cap.
Next time it cools, it sucks in more air, and blows out more coolant.
After a few cycles, you end up with a lot of air in the cooling system. Air is
not as good at conducting heat as coolant, so you get hot spots inside the
engine. This often leads to blown head gaskets. (And a blown head gasket leads
to exhaust gas in the cooling system, making the problem worse.)
Having gone through this recently on a 1991 L series, I have been talking to
lots of repairers. Several of them volunteered that if a Subaru engine has
been overheated through loss of coolant, it is almost certain that the head
gaskets (and even the heads) are damaged.
Bad luck if this is the case. Make sure next time that leaky hoses are fixed
ASAP.
David