Watch this video from This Old House to learn how to repair a pressure assisted toilet.
1. Turn off water to toilet by closing the shut-off valve.
2. Flush the toilet to empty tank.
3. Disconnect water-supply line from toilet.
4. Unbolt tank and lift it off toilet.
5. Use pliers to take off spud nut from bottom of tank.
6. Unscrew nut holding fill tube to bottom of tank.
7. Slide the pressure-assist unit out of the tank.
8. Use a utility knife to cut off the defective pressure regulator.
9. Install new pressure regulator and attach with stainless steel hose clamp.
10. Reinstall the pressure-assist unit and bolt on the toilet tank.
11. Turn on water and flush toilet to test new regulator.