ZL-1 and Power Brakes

[wvhylander]

I had Comp Cams grind a roller for my BBC specifically with power brakes. They said for good vacuum numbers I needed to stay under 250 degrees duration @ 50. The cam came in as 236/242 547/567 lift. I've got vacuum but not the hp numbers I was looking for. So my question with the duration and lift numbers I've seen on ZL-1 cams how did GM get vacuum to operate power brakes? I've seen duration numbers listed @ 260/270 @ 50. I know it's is not as simple as looking at duration, but Comp Cams didn't grind a cam with numbers close to the GM cam. They offered to regrind closer to 250 duration, but no guarantees on vacuum numbers.