soft brakes and brake light comes on-help[

Ok- 74 duster with disc up front and 10 inch drums in rear. I am having issues with my brakes being soft. I put on a new-rebuilt- master cyl about last summer?? Have good brake pads and brake shoes. No visible leaks anywhere and fluid doesnt seem to be going down at all. Bled the brakes-again- this past weekend. Helped very slightly-may just be the feeling of wishful thinmking also. They are power brakes. Im guessing either a bad power brake booster or somehow still have air in the master cyl.
Any ideas? Would a bad power brake booster give you the feeling of soft brakes? Making the brake light come on? I dont seem to get any air out of the lines when bleeding them.