Pull the vac modulator hose off the modulator. If it is leaking past the modulator the hose should be wet inside with ATF, mabe even full judging from the amount you are losing.
Look at the coolant to see if there is oil in it. You may have to disconnect a hose to check it if your car has a coolant recovery tank. If it has oil in it the coolant should be milky looking, maybe with beads of red mixed in not clear green like antifreeze.

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Darrell

1984 300SD "Ole Girl"
207K