Lost what feels to be most of, but not all, the power brake assist.

I hear a slight air leak (vacuum?)noise when I hit the brakes but i think it did this before but now it seems different in that it persists after brake pedal depression is held steady at a given point.

Is this normal to still hear air(presumably vacuum) leaking when depressing the brake pedal then holding at a given position?

If abnormal then i presume i need a new booster..if not, what else can i check and how do i check the booster?

I did start it and then let it sit for a few minutes...then pulled the check valve and listened for air rushing in.. after 1 minute air rush was significant.. after 10 minutes it was noticeably less.

Cardone remanufactures boosters and sells them at Autozone for $80 with lifetime warranty.
They also sell ARI brand (also lifetime warranty) fro $125 but the warehouse is out of them and I dont know if they can get them.

Anyone know if there is a difference in quality?
Anyone know of a better source?