If you are not sure what you are doing it is possible to depress the brake pedal too far. This causes internal damage to the Master cylinder. The result is one half of the master sylinder works, the other half tries to pressurize the resevior on the up stroke. This may be what is causing the bubbles. Only solution is to replace/rebuild the master cylinder. Happened to me as I was using a new helper to bleed the brakes. good luck