You're not interested in the vacuum modulator.

You're interested in the vacuum control valve. This shall henceforth be known as "VCV".

This valve is white in color and sits atop the IP. It has a vaccum line going into the top of the valve with a green damper inline. You'll see this green damper and will know that this is the valve.

The line with the green damper is connected to the main vacuum line with a T. One side of the T goes to the supply vaccum and the opposite side of the T goes to the transmission.

What you will do is to remove the supply vacuum line and connect up your Mityvac. Now, you've got a Mityvac on one side of the T and the transmission on the other side of the T and the VCV on the bull.

Pump up the Mityvac and see if it holds vacuum. I presume that it will. Get a reading and report back with the result.

Watch the Mityvac and slowly open the rack. The Mityvac gauge should fall gradually the further the rack is opened. When the rack is fully open, the reading should be close to zero.

All these tests are performed with the engine not running.

Report back with results.