First of all
IT IS BRAKE not break. The brakes stop your car, if they don't you will likely break things when you crash.
Sorry, pet peeve, lol.
To bleed a master cylinder you are looping the output back to the brake fluid reservoir. As you actuate the piston inside the master cylinder you are forcing any air out of the master cylinder. The loop back just directs the output back to the reservoir so you can not make a big mess and reuse the fluid to bleed the master. Once there are no more bubbles in the fluid coming out of the loop back tube you should be able to connect the master cylinder back up as normal
Can you do it in the car? Yes, but sometimes the fluid likes to squirt up out of the reservoir if you go too fast/hard with the pumping. Then you need to bleed the rest of the brakes.