Question on brake booster, 66 C20

[earl84]

OK, I have a 66 C20, with drum brakes on all 4 corners. I bought it with no brakes at all. Replaced all brake hardware on all 4 corners, all rubber brake hoses, and did the splitting of front and rear with a new drum/drum master cylinder for a '72 c10 (if I recall correctly). I have fluid at all 4 corners after gravity bleeding, but when I had my daughter manning the brake pedal to do a bleed, the pedal goes to the floor with no resistance and there is a loud hissing under the hood, seems to be coming out of the intake. It is not acting like all the other vehicles I have bled brakes on in the past.
I believe I have a bad vacuum booster, but not sure. Hooking up my Mityvac tool to it, it will get a vacuum, but takes about 60 pumps to get to 15 inches of vacuum, and it bleeds away after about 2 minutes. The check valve is working correctly, and it is a remanufactured master cylinder, and I measured the pushrod and it was set to the proper length.
Any thoughts? Was hoping to get it on the road today, but still no brakes, so that won't happen.
BTW, this is a GREAT forum, with some GREAT folks.