I'm afraid you are. The space in front of the diaphragm is the vacuum chamber. That "dust boot" is actually the seal between the M/C and the booster that ensures vacuum is maintained. Note that the seal has a sort of square cross section "o-ring" molded into it's outer diameter. That seals the back of the M/C to the front of the booster.

 

I bet you're right, Joe. Now that you've explained it I bet that's all it was--the small seal behind the MC. I already replaced the booster, however. Maybe just as well, it was really rusty and corroded from a bad MC leak that the previous owner let go for years.
Is there any value in keeping the old booster for a future rebuild? Or should I just trade it in and get my $15 bucks for the core?