2001 Chrysler Van

We have been 6 times to our repair shop with air in the brake lines they have tightend everything replaced the front right caliper only to have about three weeks later to have air back in the lines. We are very frustrated and worried about this. Wondering could this be the master cylinder that we are having problems with there are new brakes on the vehicle. Thank you my husband has been off work several months we need to know how much expense this is to fix or can he do this himself. Thank you.