If linkage adjustment does not improve braking, open them up and check for bearing leaks - they don't work too well if they're all fouled up with grease and oil. Also allows the check of the linings themselves - I had the linings on one shoe crack up and completely separate from the shoe. The drum had become rusty and rough from water infiltration. My symptom was the characteristic metal on metal grind. Had the oil seepage/slippage problem on a Ford 600.
If needed, replacements are pretty easily found - cheapest I found were: used at KY Tractor; new on ebay.
YMMV
Bob