I also boil my skulls. First of all, try to clean as much of the skull by hand. When your water is boiling, mix in salt and possibly even Oxi Clean. I do this and it seems to help loosen the hide and meat faster while also whitening the skull some even before it is time to bleach. But like some other posts stated, try not to boil to long, it does soften the skull and can break easier. One way to help this problem is be sure to dip the skull in cold water after boiling the skull. Try some of these methods and see if they help.
HuntinGuy