4wd vacuum question

[Sampson]

I have a 1988 GW with select-track 4wd. It will not shift into 4wd. The transfer case will shift manually if I disconnect the vacuum motor. The vacuum motor works correctly on the bench. The vacuum switch seems to work correctly. The switch will move the vacuum motor into 4wd position and will return it to 2wd if the vacuum lines are connected independently. (IE: vacuum line from 4wd side of switch to the motor connected to 4wd side of the motor but no vacuum line connected to the 2wd side of the switch.). However if both 2wd and 4wd vacuum lines are connected the motor will not shift. It acts as though it does not relieve the vacuum from the side of the motor that is being deactivated and maintaining vacuum on both sides of the motor.
There was a 1 way check valve in one of the vacuum lines when I removed the transfer case from the GW. I am not sure how this was connected in the system but have tried to reinstall it in every way I can think of.
Is there an exhaust port on the switch or vacuum booster that could be plugged or inop? Any other thoughts would be appreciated.

[Tripwire]

there is no check valve that i recall, i think your dash vac switch is leaking and causing it to stay engaged ?

steve

['89_Wagon]

Cracks in vacuum lines? Maybe try applying vacuum at the lines that connect to the switch via vacuum pump and see what happens? Should be easy to get to. Might indicate a leak. Switch is held on my 2 bots.

[Sampson]

Thanks for the reply

[babywag]

Don't rule out the vacuum motor being "weak".
It's over 30yrs old now, it may function correctly on bench or disconnected from linkage, but may not have enough oomf to actually shift the case.