I've been getting fantastic results with a home-made brass cleaner that uses citric acid as the cleaning agent. You can find citric acid sold inexpensively at health-food stores, and some drug stores, farm-supply stores, etc.
The citric acid powder is mixed 2 teaspoons to a quart of water. The exact concentration does not matter too much. A half-ounce in a gallon of water also works. High concentrations are not needed. Citric acid is the active ingredient in home-made brass cleaners that use lemon juice. Same stuff.
To use, pour enough boiling-hot water into a large glass (or non-metal) bowl to cover your dirty brass. Add the citric acid and give it a quick stir with a non-metal spoon. Then dump in your brass, and stir or swirl the container for a couple seconds. You will see the tarnish and dirt being removed almost instantly. Usually takes only a few seconds to a minute or so, and it's done-- there is no need to soak for a long time. The brass will look almost like-new, cleaned inside and out. Rinse the brass with clean hot water, and set aside to dry. You can tumble to polish the cases after drying, and brass treated with this solution polishes quickly. To reuse the solution later, just reheat it.
The advantage of citric acid is that it will not penetrate or damage the brass like ammonia-based brass cleaners will, and it works even faster. Also, citric acid passivates the brass, which means that after washing in the hot solution, the brass is actually made more corrosion-resistant. If you store brass for long periods, that's great news.
Below is a photo of piece of dirty, tarnished 50BMG brass that I dipped in the citric acid wash for only a few seconds. It cleans like magic. Give it a try.
Thought this might be of help to someone. And hey, why don't we have a forum on BRASS?