FACTS:
The car is my '65 Satellite which I converted from single to dual circuit brakes (all drums). I installed a bench-bled remanufactured Bendix MC, the original pushrod and pedal. All the brake lines are new from InLine Tube. The three flex hoses are new. None of the brake cylinders leak, but the fronts are new & the rears were the ones I bought the car with. The brass distribution block is from a '67 Belvedere and I installed correct hydraulic nipples at the MC, spiraled the lines to the block, and double-checked the install against Ehrenberg's tech article in Mopar Action April 2004.

PROBLEM;
If the car has sat overnight and I take it for a drive, the rear driver's side wheel locks up really really easily. Scary. The rest of the brakes have little action, if any. But after about 4-5 full stops, this problem goes away and I can spend the rest of the day with completely normal braking, evenly balanced on all four wheels.

I'm cautious during those first few stops. I come to a full stop and pump the snot out of the pedal. After the first few stops, it brakes evenly without lock-up even under heavy braking.

What on earth could cause this?

Something changes or is corrected in the first 5 minutes of driving and braking?

The car can sit for hours after that first drive, and will still be fine. Some air "bleeds back" when sitting for 8-10 hours?

NOTES:
- The pedal does harden up a bit when I pump it, meaning there could be some air somewhere, but I've bled the @!#^%@ out of the MC and every line, in the correct sequence.

- After a good drive and I can feel that the rear wheel temperature is hotter than the fronts, which tells me that the rears are working more (or dragging?) than the fronts. Why? Don' the fronts "work harder" than the rears?

- The shoe-advance mechanisms seem to be working fine. But this is something I could double-check. (I had asymmetrical rear braking on my '67 which I traced to a stuck advance mechanism, but that was a permanent condition during a drive, it didn't go away after the first few minutes).

IDEAS:
1. - Check the rear shoe advance mechanisms.

2. - Check all four brake cylinders? - nothing leaks but I replaced the fronts with new. The original rears were fine, so I cleaned them up and reused them. These are one difference betw front & rears.

3. - Could air be trapped in the front brake MC circuit that goes away? I've already had the MC out once to pump and re-bench-bleed. PITA. But can air still be trapped in the MC after 400+ miles of driving, and bleeding of the brake lines?

4. - Take the car out tomorrow and, when the brakes have evened out, brake until I get a lock up and see if the rears or fronts lock first, just to get clarity how well the fronts are working. Kinda pointless, but it might reveal …something?

Thanks for reading and thanks for any insights.
- Art