I don't have a grand prix, but I would hazard a guess you "should" be able to get at least two actuations out of the vacuum resevoir with the engine off. There is probably an internal check valve on the vacuum can that is bad or a leak in any one of the lines or switches. You could try and install a universal check valve on the engine side of the vacuum resevoir. The other thing might be higher lock mechanism resistance due to age requiring more vacuum than when new to operate. You could tee in a vacuum gauge to check what the vacuum of the lock system does when the engine shuts down, or if it maintains what the draw is when the lock switch is pressed.
My '67 T-bird has the vacuum door locks too and I only get one actuation after the engine is shut off. I attribute this to high resistance in the lock mechanisms as the locks wiggle just a bit but won't click over.
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"How I learned to stop worrying and love the OHC Pontiac L6"
The Silver Buick- '77 Skylark coupe w/455, SPX, MegaSquirt 3 & TKO-600 (Drag Week 2011, 2012 & 2015!)
1969 Firebird with a turbo'd Pontiac L6 controlled by a MegaSquirt 3 and backed with a microsquirt controlled 4L60e and 4.56 gears! (Drag Week 2018!)
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