You must have some sort of flexible dipstick tube?? If so just look at it to see what it is made from. If it has a rubber liner then it should be air tight but it sounds sketchy to me for sealing. I guess it just depends on how much vacuum you are trying to pull. A dipstick will always be a problem unless you get one that is designed to seal. Older Mopar engines were not designed by the factory to be sealed up so it takes some work to find and fix all the leaks.