Seems like 15 inches should run the brakes...but it’s been a long time since I’ve had to deal with that. Seems like once you got down to 10ish inches of vacuum was when things got sketchy.
Same here. Last power brake vehicle I built a engine for had a pretty mild cam (20" at idle) for 1/2 ton 4x4 truck application. I think booster size matters too, small boosters might be less efficient.
