Prepositions or Conjunctions: after, before, since, till, until

The topic of the present paper was suggested by an article by S. Greenbaum “Adverbial -ing Participle Constructions in English”, where the author, when describing the structure of adverbial non-finite subordinate clauses in English, mentions, apart from other means of expressing the adverbial function, also those non-finite clauses introduced by after, before, since, till, and until, saying that these conjunctions “...differ from other conjunctions that introduce non- finite clauses, e.g., while, though, when, in that they appear to have a distinct prepositional use ...they can take as object derived nominals (and other phrases with a noun as head) as well as gerundive nominals.. .” According to Greenbaum, ...if a derived nominal is substituted for the nonfinite construction” after these conjunctions, they are “traditionally recategorized as a preposition. {Since his refusal of the offer, John hasn’t been to see us.)”?
It might therefore be interesting to find out what makes after, before, since, till, and until conjunction-like, and which factors support the prepositional function.