Delay shift basics........

The delay system is simply an ACTUATOR on the side of the trans that , when vac is supplied to its ELEMENT, mechanically changes the position of the shift cable . [ comparable to the driver moving the gear selector to a lower gear selection]
This part of the system is not a Switch Over Valve or any other type of valve. It is a vac activated cable servo.
The activation of this cable actuator is controlled by the ECU sending an electrical sig to the SWITCH-OVER VALVE as determined by the ECU mapping. This allows engine vac flow to reach the tranny cable actuator through the SOV. [ this is the part behind the booster and is just an open/closed electrically energized valve]
As soon as the ECU is satisfied , it stops the signal to the SOV ,which then closes and vac flow ceases to the actuator, allowing the cable to return to normal shift position.

So, a code 26 can be anything from no electric sig to the SOV to a bad actuator...and anything in between. [vac hose leaks,etc]
An easy test is to take the 2 vac lines at the SOV off and connected them together with a small piece of tubing. This eliminates any ECU engine management of the delay system. Take the car for a ride and see if the actuator is responding to engine vac.
[the tranny should stay in delay mode] Check for engine vac at the disconnected line before putting them together to assure of vac at sov in-line from engine]
This simple test gets vac right to the cable actuator and you can work back from there..if you now have delay , then look at SOV flow and ECU signal to SOV, etc. If not , you have bad element in actuator, bad actuator, or [ and don't overlook this ] a broken vac line at actuator.......
Many guys just put a BB in the line at the SOV and use a lower shift range when the car is warming up [ serving the same system purpose]. But, then the CE comes on and has to be erased. [ surpisingly, sometimes this can take a month]
The reason for the BB line plug is to prevent a vac leak while the SOV is open.....