[Lynn Eades]

There are a number of possibilities for your problems.
Your bisector can be assembled incorrectly, to give a minimal amount of travel as snow tractor says. So make sure they are assembled correctly, lubed with a suitable high temp grease.
The split pins just hold them together. Think carefully as you put them together. You are looking to get the maximum piston travel, in other words the ramp on the midldle bit, sits at the opposite angle to the ramp of the outer parts.
The reason for the bronze is that the linings were too thin.
If you turn out the drums then the replacement linings must be equally thicker. Your drums may allready have been turned out (How many times?) Not to offend or disagree with what Rob is saying, but unlike current road transport, replacement drums are not easy to find for a carrier, So, I would be trying my best to clean out my drums with emery tape to get a good clean surface, rather than take more meat out of them.
These brakes work very hard, and need the "meat" in the drum wall to do the job.
You also want a lining that works well at low speed. The brake people may recommend a woven lining for this application.
Because you dont know how many times your drum may have been turned out, you need to take your shoes and drums to the brake specialists and have a lining of the correct thickness, that is "radius ground" to suit the drum.
Clean those bearings and all other parts well. Lube your bearings well, but dont pack the hub full of grease. Use new seals (if they leak the grease will ruin your linings)
Make your brake adjustments as per the book, before you fit the axles, or track.
A brake is an energy converter. It turns kinetic energy into heat.