Don, if your bellow is still in good shape, as you state, then it should not "run out of air". Here is a good test: open the accordion about 6" or so. With NO buttons pressed, apply slight pressure pushing and pulling and feel if there are any air leaks. If there are no air leaks, the bellow is fine. Air leaks can come from any number of problems, not associated with the bellow.
My suggestion is that if the bellow is not the source of an air leak, replacing it is not necessary. My 2 cents worth.
Jude