I smoked for twenty years and recently quit, I have allergies and post nasal drip. No matter what I do I can not get rid of the post nasal drip and subsequent bad breath, quitting smoking has helped somewhat but not really, I use saline nasal spray and a

Bad breath. See your ENT for post nasal drip. Meanwhile use a tongue scraper to clean your tongue few times per day because the bacterial can sit and harbor on top of your tongue perpetuating the bad breath.
Sinus infection. The problem may be chronic sinusitis. Saline irrigation with a neil med sinurinse bottle might do a better job of clearing any debris. A good exam by an ent, including a nasal endoscopy would best examine for the origin of bad breath. Sometimes your breath isn't as bad as you imagine, also.
Mouth breathing. Odors causing bad breath come from the stomach, lungs or sinuses. Your bad breath is the result of clogged nasal passages. Post nasal drip is another symptom of your nasal-sinus disease. It may be a sinus infection, nasal polyps, or allergic or nonallergic rhinitis. When your nose is blocked you breathe through your mouth. Dry mouth and odor is the result. Restore nasal breathing & odor resolves.