How to permanently seal a pipe filled with water and under pressure making it equal to the wall?
Now we are getting somewhere. You have a pressure flushing system which is malfunctioning, and I can understand your desire to remedy this.

But your solution is too radical. The first attempt should be to get a better pressure system and not to do radical surgery on your existing piping. Can't you replace the valving with a better quality one? Tank (aka cistern) flushing systems have their own problems.

Is this an emergency? Have you removed the toilet and this is your only one?