4kq coolant reservoir level

[Robert Rossato]

The coolant reservoir level on my '87 4kq drops quite a bit (around 2")
when the engine is running, and as a result the coolant level light will
flash any time I go around a corner.  When it's off the level is pretty
much at max or slightly below max.  Definitely well above the min level.
If I were to add enough coolant so that it wouldn't trigger the low
level sensor when running, it would just overflow when shut down and
hot.

This seems to be a common problem as I had checked out another 4kq
before picking up this one and the owner noted that the coolant level
light would always flash but whenever he checked the level it was
normal.

Any thoughts at what would cause the levels to drop so drastically when
it's running?  Coolant temperatures are normal.

Bob