Bubbles in the return line indicate a suction side leak. Most of the time the leak is caused by a faulty pump cover o-ring. Have a look at your pump basket and see if there is a lot of air in it while the pump is running. When you turn the system off you might actually hear air escaping around the cover. If not, remove the cover and inspect the oring. More than likely the oring is faulty.

Brad
www.waterworkspools.com