A down hole pressure isolation tool is placed in a pipe string and includes a pair of pressure discs having one side that is highly resistant to applied pressure and one side that ruptures when much lower pressures are applied to it. The weak sides of the pressure discs face each other. In some embodiments, an upper disc is mounted for movement and restrained by a shearable connection. Upon the application of pressure to the tool, the connection shears allowing the upper disc to move into and collide with the lower disc thereby fracturing both discs and allowing pressure communication across the tool. In another embodiment, spurs carried by a sleeve fracture an upper disc in response to pressure delivered to an upper end of the tool.