The term "burping" a reefer, means that you remove the reefer and turn it upside down, again and again and again.
A cooling unit goes bad, internally. Small particals of rust can get loose, circulate and then plug or partially plug an oriface.
When that happens, the flow of refrigerant (ammonia and water) is reduced which in turn reduces the cooling ability.
In my 40 years, I have never heard nor do I know anyone that was ever successful in burping a heat absorption type refrigerator, that stayed "fixed" for more than a few days.
This does not happen often, but it does happen. There are thousands of reefers out there that are 50 years old and are still going strong.
Bottom line, is a replacement or rebuilt cooling unit, or a new refrigerator.