Rear drum drag

[TeamEstep]

Well, it's been a drag trying to resolve this problem, ha ha. I have a '71 LWB that the drums have a slight drag. I had converted the manual brakes to boosted and while I had everything apart, I also replaced the calipers, pads, cylinders, shoes, springs, all brake hoses, CPP proportioning valve and some of the brake lines. I blench bled the new master cylinder before installation, and bled the brake lines starting at the furthest wheel. The brakes work fantastic but the back drums have a slight drag. Sometimes worse than others.
Here is what I have done/investigated so far:
1)I took the brakes back apart and took my flapper wheel to the contact points on the backing plate and cleaned and smoothed them, then added a thin film of high temp grease to those surfaces. I also cleaned and put a small amount of grease on all pivot surfaces.
2) I made sure the primary shoe (shorter liner) was forward.
3) I made sure there was a wavy washer on the e-brake lever. (Brake cable is not currently connected to the pedal.)
4) I made sure the adjuster was not too tight (adjusted until tight then backed off about 3/4 turn. Wheel turned freely).
5) I made sure the correct hold down springs were on the right shoe.
6) The brake pedal has free play at the top of the stroke so shouldn't be pre-loading.
7) The master cylinder/booster came as a kit so the rod length between the master cylinder and booster should be correct.
7) I checked the level of brake fluid in the master cylinder and it is about 1/2" below the top so there should be enough room for return fluid.
The symptoms seem to indicate a bad brake hose but I had replace all of them.
I did happen to run the master cylinder out of fluid while bleeding the back but refilled it and re-bled the back.
Could air somewhere in the system cause this symptom? Seems like it gets bad if I get on the brakes hard. If I keep brake pressures light, it doesn't seem to get bound up. Sometimes if I give it a light tap or two on the brake pedal it seems to free up but could be my imagination. It does not seem to be temp related as the brakes are sometimes dragging when I first drive it.

Any help is appreciated. Thanks!