You are going about it the right way. You will have to pump till your arm is sore to get the reservoir pumped down very far and its possible to wear out a MityVac! BTDT so I use a special method, its very simple:
You tee your mityvac into the line running to the reservoir using a one way valve (blue or green cylinder with two hose fittings) ahead of the reservoir (towards the vac source), that way any leaks up stream will not show up. Run the engine to pump it down then shut it off and see if it changes at all. If it shows any drift off vacuum then its a leaking reservoir. And there are several reservoirs as I have noted in the wagin its in a different place than a sedan as I recall. Have you checked the fuel tank filler actuator and the trunk lock? they can be leakers too. Also did you plug off the pods under the dash? That is the bad place to fix! The small vac valve on the back of the key switch can be a source of leaks. I believe I have one if you need it.

The locks should be surprisingly quick when the vac system is tight. Slower means more leaks. You should be able to leave the car about a week and still get the doors to unlock.

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'95 E320 Wagon my favorite road car. '99 E300D wolf in sheeps body, '87 300D Sportline suspension, '79 300TD w/ 617.952 engine at 367,750 and counting!