1999 Other Plymouth Models Air in the brake line again

I had my brakes replaced and had my lines bled and had brakes for a week. Returned to only have the brake lines bled again, one week later I had no brakes again. I brought it in again and they said there was air in the line again. The master cylider is not leaking and they told me they have seen this problem in Plymouth Grand Voyagers recently. Could you please tell me why this is happening and how to fix it, besides going in every week to have the brakes bled. Thank you.