No way you should be snapping pressure plate bolts off. A couple things come to mind. First, are you using the correct 'shoulder' bolts? Are you sure they're grade 8 bolts? And like Chris said, they should be drawn up evenly. Lastly, is your torque wrench correctly calibrated? I ask that last one because many big torque wrenches, ones designed to go upwards of 150+ lbs of torque, can be off by quite a bit when using at the extreme lower end of the scale. I have several, and the one I use for stuff like that only goes to 50 ft.lbs, so 25 puts it right in the middle, where it's typically most accurate.