Your brake booster/master cylinder may be adjusted incorrectly. If the rod to your brake booster is adjusted too long, it won't allow your master cylinder to return to the "at rest" position, which will block the compesating port in the master cylinder. This allows the apply pressure in the system to returm to the resiviour when the brakes are released, and also for the brake fluid to expand as the fluid heats up. If not equalized, the added pressure is the same as applying your brakes. To find out if this is happening, when the brakes are sticking, loosen off the two nuts holding the M/C to the booster, 5 full turns each, and then see if the rear brakes release.

The replacement metering block/proportioning valve may not be calibrated properly for your Camaro.It may even be for four wheel discs.Try reinstalling the earlier one,do a thorough brake bleed,and try it again.

Check your backing plates. There are flat pad areas that brake shoes slide on. Did you lube those pads. Top, middle, and bottom of each shoe. And are those pads flat smooth without any grooves? If pads are not smooth or lubed the shoes will hang up and not release correctly.

Emergency brake cable needs loosening up. Happened to me once

Re-adjust the rear brake shoe adjusters with a brake spoon through the backing plate holes.

Make sure your emergency brake cables aren't stuck partially on!

self adjust they might drag again Which is quite possible as the adjusters are not working properly. Disassemble and reassemble after cleaning and inspecting.

1967 Camaro completely restored, all new parts, MC, booster, emergency cables, shoes etc.

Brakes worked normal and good on first trial 50 miles. Now rear brakes drag and overheat causing expansion of fluid, pedal pumps up hard to the top when you apply them, brakes lock up in about 5 miles. Thought the MC push rod might have been too long, not allowing the fluid pressure to vent in MC and escape, so shimmed the MC back in stages of .047" up to 3/16". This allowed the pedal to have more travel and extended the distance between overheating lockups to 10/20 miles but it still happens.

When the lock up occurs, sometimes the pedal will come back to normal after waiting 20 minutes to cool, sometimes it will stay locked. I can always get the car moving normal again by loosening the master cylinder nuts and shifting it off its mount and re-installing, pedal goes back to normal. This would suggest the rod is too long still, but I can hardly believe it. The MC, Booster, push rod 3.972" long is all from the same restorer brake supplier, short pushrod type with 1" bore MC.

Disconnecting the vaccum from the power booster doesnt seem to make a significant difference.

There may be two problems, One is why does the pressure not vent to MC from the lines when fluid gets hot when not using the pedal? Why do the brakes drag to begin with?

I suspect I can loosen off the brake adjuster and the brakes will not drag initially, but after they self adjust they might drag again.