Even more perplexing is that you refuse to admit that mixing a breed by breeding something into it that is not the same breed will alter the litter and cause it to no longer be the same breed.
By your definition I don't believe any dog is a "pure" breed.
I would say if you were to do a DNA test on your dog you would most likely see a mixture of breeds with one dominant breed in the DNA the Presa. However, the best dog DNA tests today only go back three generations and are rudimentary at this point in time.
The tests are always improving maybe in 3 or 4 years they will have more detailed ancestry.
If finding out others breeds were introduced to your cleanest Presa then the question to ask is your dog the same breed?