FWIW, all known concentric throwouts seal by means of a single internal 'quad-ring'- a specialized type of o-ring. It seals both a piston traveling in and out its bore at considerable hydraulic pressure from the master cylinder, and also seals against the spinning clutch input shaft, but a single quad-ring simply cannot seal its piston rotationally as well.
So as was said, sufficient free play is vital to absolutely preventing the throwout bearing piston from bottoming and spinning with the clutch. Once that happens, the game is soon over for that quad-ring. I installed a ZF-lube temp gauge sender near the ring gear, and on a 'spirited' 500-mile desert road trip I saw only 190 F max. Racers have seen considerably more, hence the heat expansion problem. Leaving the stock ZF ventilation hole open reduces internal clutch heat.
When they work, they're an improvement over stock multipart hydro-mechanical linkage. When they don't, they're far more trouble to fix. With all concentric throwouts, the transmission/transaxle must come out or at least be slid back enough to change or repair any leaks. You need to run a flexible dash-3 hydraulic clutch line into the bellhousing and a second line out with a bleeder valve at its end. Empty ZF cross-shaft holes work- no drilling needed.
Incidentally on Tiltons, there's a large OD secondary o-ring that functions as a dust-ring in front of the throwout bearing itself, to help keep abrasive clutch particles out of the piston bore. I have had one of those fail, but it didn't result in a fluid leak. It was easily replaced when the ZF was pulled for other reasons.