If it does not hold vac, then there is a vac leak between the Switch-over-valve and the shift element. It could be as simple as a bad/broken rubber vac line down at the element, but most are a ruptured element.
Or if it does hold vac., then it is possible the SOV is not getting an electrical demand sig from ECU or the SOV is bad.
A quick trick for that is to take the in/out vac hoses off the SOV and put them together...this eliminates all the ECU commands and SOV as possibles and gives direct engine vac to the delay shift element...you then take the car around the block..it should stay in 2nd/shift delay mode b/c you are direct feeding the element w/o engine mangement or SOV control.. if NO , then the shift delay holding element is not working.[ providing you know you have engine vac going there]

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A Dalton