Rear brakes locking up to fast

[Lee H]

Well guys, I'm having an issue with my braking system. 1972 power disc/drum. The rears are locking up rather quickly. Seems little to no front braking action. When I freshened up the truck last year I replaced "all" braking components, hard lines, rubber hoses, all parking brake components, drums, rotors, wheel cylinders, calipers, booster, master, proportioning valve. I originally thought it was a mismatch between the master, booster. I tried an adjustable push rod and no change. I purchased a master, booster, proportioning valve kit from CPP, no change in rear lock up. The pedal travel is right on spec with the service manual and is nice and firm. I loosened up the parking brake cable with no change in rear lock up. No sign of brake fluid or gear oil. There are no crimps or kinks in the hard lines. Don't know if this is recent occurrence but I suspect it has been around for a long time and a recent aggressive braking action brought it to my attention. The rear shoes are nearly worn out after just 8K miles.

So, here's my next steps:
Replace the rear shoes and hardware so that is up to snuff. Trying to find a good detailed picture of a correctly assembled drum setup. The pictures in my service manual are not all that clear. Maybe I do not have all the components installed correctly.

Next will be replacement of the calipers & hoses. All summer I thought the lack of brake dust on the wheels was due to good pads but now I think that is just a sign of the real problem.

Advice is appreciated.