How do you unscrew a stuck valve on a marching tuba?

To unscrew a stuck valve on a marching tuba, first ensure the tuba is clean and dry to prevent further damage. Apply a small amount of valve oil or penetrating lubricant around the valve casing to help loosen it. Gently twist the valve back and forth while pulling upward, being careful not to apply excessive force. If the valve remains stuck, consider using a rubber mallet to lightly tap the valve casing while twisting to help break the bond.