This is your vacuum diagram.

It can flare for one of two reasons:

1) Item 63 is missing. It is imperative that this restrictor be present otherwise the trans will see too much vacuum.

2) The adjustment of item 65, the vacuum control valve, is incorrect. Disconnect the line to the vacuum control valve and connect the Mityvac. Pump up the Mityvac and see what you get at idle (engine not running).

Then, open the rack and watch the Mityvac. The gauge should fall gradually toward zero. If it remains at high vacuum, the VCV is the culprit.

If the modulator is leaking slightly, you'll have to continually pump up the Mityvac to the leak point........be careful with obtaining data under this condition.

You may wish to connect the Mityvac directly to the transmission line, first, and pump it up. See if it holds vacuum before continuing with the aforementioned test.