Air in brakes????

I've been working on cars a long time and never had this problem,
69 chevelle ft disc brakes, NEW! master cyl, two weeks after installing and bleeding with pressure bleeder, air in left front caliper, put on two rebuilts before the new one.
No leaks, no loss of fluid, have to bleed every two weeks, only the left front! where is the air comming from? HELP!!!! (please) thanks

This might be kind of "stretching" it, but maybe the brakes are biting down too hard and causing the fluid to heat up (boil/bubble)? Any burnt smell in the pad/caliper area? Maybe the piston is sticking in the caliper and not releasing pressure from the pads, causing the overheating? Since you're not leaking, can't be the seals or lines. Since the problem is isolated on that one side, I'm guessing it's the caliper assembly.

Thanks
nothing dragging or sticking, I was thinking maybe the metering or proportioning valve but i don't think there are any moving parts inside to cause aeration are there?