SLKPK, there is a clevis on the rock shaft end of the brake linkage.
While I don't have one in hand to verify, I believe this will tighten it: unpin the clevis, turn it a few turns down the shaft (shorten the effective shaft length), then repin the clevis to the rock shaft. This should tighten up the rear disc brake.
This brake mechanism is completely different than the garden tractors, featuring a small mechanical disk brake on the hydro unit. However, the linkage is actually rather similar to the big tractors and I think this should work.
Even so, I would take a quick look under it to ensure nothing else appears out of place before getting in to adjustment.