Brake vacuum

[forestb]

I have front disks, rear drums with power brakes and an automatic transmission I recently installed a new motor with a more lumpy cam. I checked my vacuum at the hose that comes out of the back of the carb and at idle it fluctuates between 10 and 12. When I drive around normal it works fine but my parents house has a vary steep driveway and when I am backing out I am not able to bring it to a full stop until I hit the bottom. Is that enough vacuum. If I remember correctly 12 is a good amount.