The ID number is required to look up the exact model, but I think your setup is the same as most of the era with a horizontal shaft engine.
The brake pedal connects to a rod that pulls a band tight on a drum for braking action, with a spring extending back from the band and hooking just above the axle. The rod has an adjusting nut on it where it passes through the band.
If the pedal didn't come back up, check to see if the spring broke.
The brake is adjusted by simply tightening the nut on the rod, however, make sure the brake lining inside the band is intact before just cranking down on the nut. If it's badly worn, or oily because the seal behind it leaked, you'll have to pull the drum and replace the seal as well as replace the brake lining. I got some flexible brake lining at a local auto parts store.
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C-Horse ( C-141 + Commando 800)