Consuelo, I find this very interesting.
To me (and this may just be my personal use of the terms):

blowhard or windbag or hot air or blowing hot air imply ineffectual, empty rhetoric.*
letting off steam implies rhetoric primarily to vent one's feelings, though the rhetoric may seek actual persuasion as well.
breathing fire implies "fighting words": the swords are drawn.

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*as in the fact that the sobriquet of "the windy city" for Chicago was originally coined to refer not to the city's meteorology, but to its aldermen.