Here's my two guesses:
1) Are you running glycol or silicone based fluid? I don't know if they use 'DOT' ratings for fluid down there but there seems to be confusion in the types of fluid and some people still insist on using silicone based fluids. There are many problems with silicone fluid which I'll spare you but the biggest problem is that it absorbs air bubbles that won't come out until the fluid is heat-cycled. I used to run the stuff many years ago but completely gave up. I was constantly bleeding the brakes after a change. It took 3-4 bleeds to get it all. Make sure you stay with a glycol based fluid and you shouldn't have this problem.
2) It may not be 'air' but rather some other kind of gas being generated by active corrosion inside your caliper or master cylinder. As it corrodes, gas is given off and collects inside. Clean fluid won't allow that to happen but once it starts, it might be your source. In such a small caliper, a little gas goes a long way.