I recently got my 68 Camaro runnning...went with disc brakes in the front and left the drums in the back for now...upon driving the car I noticed that as I shift to neutral and let the car coast, it slows itself down, rumbles the front end and ultimately feels like its braking. Checked the front calipers and they appear to rotate fine as well as the rear drums. Help! Cant figure out what might be causing this

 

Check the rear brakes. With the rear tires off, the drum should turn using two hands with just a little bit of drag.

 

Recently I had my back brakes drag. I tried a number of things then replace the master cylinder. That solved it.