Adjustable brake push rod

I just remembered I had a similar problem with a 71 Challenger I had back in the 80s. I converted it over to front discs (used all factory stuff from a 72 Satellite Sebring) & it would slowly but surely start dragging the front brakes, the pistons were not returning all the way. The pedal was good & firm, but it ended up having an air bubble trapped in the brake system. I re-bled the brakes & that fixed it. So, it's possible that you still have some air trapped in your brake system also.