If nothing shows up along any of these lines, think about corns. These are not unheard of, especially in the animal whose claws are long, deformed, or slightly crossed. You'll see a finger-like projection of callused tissue protruding from between the 2 claws in front, at the coronary band. It can affect either cow or bull. Treatment involves paring the corn and trimming the hoof to remove the situation that started the corn in the first place.
Severe cases cause enough irritation to become infected.
These may require surgical removal of the corn. It'll take the old bull a while to recover, so it's best done out of breeding season. Successful treatment of lameness problems boils down mostly to good hoof care, and catching difficulties early, when the response is rapid.