what about just leaving the system alone and converting to disc?
That is a great question!
Safety. The primary reason is safety backup. With the Stock system any failure in any of the following ends in catastrophic failure: MC seal failure, Air pack issues, hard-line pipe corrosion, soft-line flexible hose deterioration or physical failure, and wheel cylinder failure. Not to mention there are several check valves.
By adding dual circuit brakes you are providing redundancy in the key areas by basically splitting the brake system into two parts rather than one. The single output master cylinder is replaced with a dual piston unit that has two pistons one in front of the other, where one drives the front brakes and the other the rear. This one modification is key for safety.
Also being covered in the thread is adding a hydro max or hydro boost that removes the air system from the brakes that is a corrosion point in most rigs and places the boosting of the system to a hydraulic loop run by an added power steering pump or other hydraulic system pump. The hydro max systems go a step further and adds an electric driven pump as a backup to this hydraulic system, giving the boost of the system a safety in redundancy as well.
All in all, safe is key.
Disc brakes are great, but they would only replace the wheel cylinders in the system and not the other 80% of the failure points.
Having had a single circuit system in my 1966 FJ40 and having had it fail on "The Lions Back" in Moab Utah I will have dual circuit brakes just because of that one day. Your welcome to look it up and tell me you wouldn't be scared not having brakes on that sandstone fin. :O
Thank GOD that an Angel spoke to me in a clear calm voice and told me as I was nearing the half way up point..." BACK DOWN". When I took and put the trans in reverse, I pushed on the brake peddle to slow my decent and the brake peddle went straight to the floor with ZERO pressure. My transfercase rear mounted park brake only worked... "kinda" going forwards. My tranfercase and trans were worn from V8 power and under tension in reverse would pop out of gear after clicking a few times. It popped out about 30 feet from the bottom. Quick driving skills kept me and my friend safe. If I wouln't have listened I can only imagine what would have happend when I went to make that 4 point turn around on the rounded top of the Lions Back.
I will not have single circuit brakes for long on my M813. Since that experience brakes are Safety #1 and I am sure in Utah my M813 will see some steep mountain sides.
Be safe out there.