They're really expensive and pretty troublesome. That's why a lot of people go to vacuum conversion or Hydraboost brakes on TB's. Between accumulator balls and other PM problems, most people just raise the white flag and do something else. There are still some people running them. If you're doing a concours restoration, you should probably just spend the money. If you really want to drive it a lot, I would do something else. I still have my original in a box that says "DO NOT OPEN, GIVE TO GUY THAT BUYS THE GN AFTER I'M DEAD". It will never go back in as long as I own it.
I just got tired of having the brake light come on and knowing I only have one pump of the brakes left. Sooner or later I was gonna wreck it. Oddly enough, my Saab 9000's have a very similar set up, and they never fail. Almost looks like the same accumulator ball. I wonder if it could be retrofitted?