Q. My Pitbull has a big ball on the back of his ear and it's got puss and blood coming out of it
A. Sounds like an ear Seroma or hematoma. These are caused by excessive shaking of the head which then damages the ear cartilage. Usually these are due to ear infections, mainly yeast, that causes a feeling of discomfort and so they shake their head and the ear cartilage is damaged. You need to see a vet to have it properly drained and sutured so it does not recur and also to have the ears checked out for infection. Or it could be an abscess that is draining, either way you need to seek vet care.