Gentlemen
Anything removed, must be replaced/resealed, so here "Less is More". All that needs removal is the bomb.
Take your new bomb to the tool store so you can buy the correct size strap wrench. You want one that is driven by a 3/8" extension. Then you'll need a foot or two of extensions, and a good 16-20" flex bar. Place a heavy towel/cardboard/blanket.....mattress...on the windshield. Caution. The Bomb may have some residual pressure within. Before doing anything else, (Engine off) step on the brake pedal several times.....until the pedal feels very stiff/dead.
Each step releases fluid from the bomb thru the pressure regulator the bomb is mounted on.
Once empty, the bomb can be "cracked-loose" and spun off. Mount the new O-ring onto the new bomb, then put the protective cap back ontop a bit. Leave it there until you've succeeded in getting the new bomb into position beneath the regulator. Then off with the protective cap, and spin into place.
when it feels really tight, "snug-it" a final tight with the same "strap-wrench" you used to loosen it.
Best of luck.

Dan Patzer
the CyberWrench
from Bimmers Only