The break I'm using comes from McClintic.....
9701 Trumbull SE
Albuq. NM 87123
505-292-1311
McClinticrdm@aol.com
basically what you have is an aluminum cylinder that screws apart. Inside is a plunger sitting on top of a spring sitting on top of shims...the shims control when the break will open. Reading their brochure, the shim is .025 thick, and each shim is approx 2.5 inches of vacuum.