This is the same problem I had, I don't have a guage, but when it go dyno'd it read 4psi.

Alpine is sending me a new pulley along with replacement neck. I'm going to measure both pulleys(should be 52mm for 5psi) and see what's up.

As for a vacuum leak, slipping belt how do you correct and spot that?

and would something like that show up on a dyno?