Have you tried to adjust them, since they are mechanical and have no hydraulic assist, as they wear down the peddle will not exert as much pressure on them if the stroke is too long.
Look for a square pin sticking out of the backing plate and adjust it until the brakes get tight then back off 1/2 turn.
Remember that mechanical brakes were never the best system but they should work "ok" at the speeds a tractor travels.