Pyrex, like all glass, can be scored and broken by hand. However, for large diameter tubing this rarely produces a straight break, and this method requires a significant length of glass on either side of the break for leverage, making it suitable only for breaking down long tubes into usable pieces. Due to its low coefficient of expansion, Pyrex is also difficult to crack with a hot wire. This leaves a diamond saw as the preferred method of cutting this type of glass.
I made such a saw following Ron Soyland's guide, using a medium-sized DC motor, a cheap lapidary blade from Ebay, and a wall-wart type 12-volt power supply. The arbor is made of Delrin, and is fixed to the shaft of the motor with a set screw. Currently I use it by holding it against the compound slide of my lathe (seen above), but in the future I may build a base and an arm for it to make it more convenient to use. It works well, and is far superior to all previous methods of cutting glass. The one downside is that the water used to cool the blade and collect the glass dust (applied to the cutting area with a wet paper towel or sponge) tends to spray, leaving glass dust residue where it evaporates. I may build a blade guard to catch this in the future.