Leak down test is good. But, you still might have a leaking head
gasket. Cylinder operating pressures are much higher than any
leakdown test. You could be leaking pressure into the cooling system,
forcing the fluid out.
Radiator cap could be bad. A new one is a cheap way to find out.
The expansion tank should get fluid from the radiator when the fluid
expands. As it cools, the system should suck the coolant back from
the expansion tank.
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