As with the locking jaw, this is urban myth.
The application of a bit of google-fu reveals that a pitbull has a bite that generates 200-250PSI. By comparison, my Great Pyrenees can be expected to generate 500-600PSI. And they're not top dog in the bite department (see what I did there?). There are breeds, such as the Kangal, that generate 700-800PSI. There are easily a couple dozen breeds with more powerful bites than the pitbull.
For reference, the average human generates about 150-180PSI and a Chihuahua (we're looking at you,
@Gyakuto) about 180-200PSI.