Our twenty-four year old tandem has a Phil Wood disk brake set up as a drag (third) brake on the rear. While it has given reasonable service, I would never use these brakes without another system. It's not that I am worried about catastrophic failure, that is a pretty rare event as far as I can tell, so much as it just seems to fade badly when it heats up. I'm just not inclined to go down a descent if I cannot trust my brakes to keep me safe.
I would advise you to value your stopping ability higher than a pretty paint job and put some proper brakes on.