OK, did the test by disconnecting all other vacuum tubes from the vacuum pump and connecting just one tube directly to the shut-off valve.
Same result as when trying to shut off with the key; lever goes down to horizontal position, but the car does not shut off (I need to manually press it further down).
I guess this would confirm the actuator is bad!?
Can it be rebuilt or one needs to buy a new one?
I checked and they seem to be going for around $200, so not really cheap!