Putting it back together is just the opposite. Make sure you verify the brake lamps work when done, since the brake switch was removed!
Please let me know if you have additional questions!
Did you remove the four nuts on the inside of the firewall?
Did you disconnect the pushrod from the brake pedal?
Thats the only two things that may prevent it from coming off the firewall! Is it stuck to the fire wall, or will it pull away but not come out? The more info you give me, the better I can help you!
Let me know!
I just did another booster on a '98 suburban (same setup). After moving the master cylinder out of the way, disconnecting the brake pedal pushrod and removing the four nuts, all it took was a slight "tap" to break it free from the firewall gasket and it was off.
If you still are having issues, I would like to help.