The engine load and throttle position play a big part in engine vacuum. I see about 18 inches (if not inches then whatever it is) at cruise in 5th, and about 20 at idle.
I could see that low number causing a problem, but I'm unsure how much vacuum the brake booster requires to fully opperate?
What are the workarounds? The strange thing was my brakes used to work fine with this cam but then all the suddenly they don't. I didn't have a guage to see how much vac I had but I see about 10. I tried revving the car up and got like 15+ and the brakes didn't seem to change.
If things used to work alright before, I'd look for an malfunctioning component (like a bad vacuum hose, etc).
Amongst the easier work arounds for the typical 'big cam' are a simple vacuum cannister/reservoir, or even a pump. The former can be pieced together for very cheap using J/Y stuff.
I'd still give things another once over since this is a new issue.