[Chester Stone]

There should be a check valve between the water filter and the tank. If there is, many coaches, including mine, have an automatic electrical drain at the bottom of the water bowl. It empties the bowl every time the compressor shuts off. In the process it takes the head pressure off of the compressor which aids startup. The 12 volt automatic system is complicated and when it failed I replaced it with 120 volt system which is much simpler. A good bubble spray is found in a children's toy department at Walmart. Children us the stuff to blow bubbles. Just add a little water and you will be able to find the smallest of air leaks.

The blue hose in your photo obscures the bottom of the water bowl.