The tank is there to provide room for the varying volume of water, based on it heating and cooling. That type of tank needs some air in it to enable that expansion and contraction, but, it also gets some disolved in the water, that can then come out in the high points. My personal opinion is that a bladder tank is better...no air in contact with the water. An air scoop like the SPirovent should help. When circulators get older, they can inject air when their seals get worn and when things cool off, you can suck air into seals that might not leak water. If you change the tank and add a good air eliminator, it should help. Depending on the design, it can be hard to get all of the air out of a system.