Other possible is No sig from ECU to SOV to allow vac flow to element.

There are several ways to test , but here is one that helps diagnose fairly fast and easy/simple....
Take the vac hose that comes from the engine off the SOV and feel/listen for vac with your thumb.. once you know you have vac there , take the other hose off SOV [ out hose to trans element]and put these two together with a small piece of tubing.. this eliminates both the SOV and ECU signal ...take the car for a short ride down the street... it should holds in delay gear. If it doesn't , the vac element at trans is the suspect/culprit.
If it does hold, then you know either the SOV is not opening [ allowing flow] or the ECU is not sending the SOV a signal. That can be tracked down with a voltage test or applying 12v to SOV and retry.
As Doc says , most likely trans cable element leaking..common problem, but try other test first...
One can also apply hand vac to line going to element and see if vac holds, but that test is not as thorough b/c it does not tell you if the element/cable are moving as they shoud.. the real time/drive test does..