The armature surfaces of all four looked good to me, not ridged or scored and clean.

 

At least on drum brakes on cars you could "adjust" them by backing up and either hitting the brake pedal or engaging the emergency/parking brake. Is that not possible on trailer drum brakes?

 

Completely flat and equally so on all four?

That technique looks good on paper but in reality doesn't work very well on any drum brake I've ever seen. The best way to adjust them is by hand.

 

That adjustment technique only worked after the advent of "automatic" adjusters. For decades prior, we used a brake "spoon" to do that job. It was something that routinely got done almost any time a car was up on the lift.

In the past few years, auto adjusters on trailer brakes have come into being, Dexter among the first. They seem to work well enough, once they are initially (manually) adjusted to get 'em close.

Pop

 

I found that I had to back the shoes off some to get the drums off when I did mine, so I would agree that they tend to work pretty well on Dexter brakes.

 

I dunno. They looked flat to me. Is there a method I was supposed to use to measure that flatness and then compare all four?

 

If you have one magnet surface that's flat, then a new magnet is going to engage and pull better than a new magnet would with a drum with a deeply grooved magnet surface. When I was dragging my brake drum all over town trying to find a replacement, I had a technician look at it and he said it was too deeply grooved to use as-is. The others were all about the same so I ended up getting three of them machined on a flywheel surface grinder. The fourth one is a new one so I wouldn't have expected to get equal braking performance out of all four tires if I had installed them as-is instead of having them machined.

 

They certainly weren't "deeply grooved". I could see or feel no grooves when I was cleaning them, and I was pretty careful with that, so I don't think that's my issue.