Defrost Help!!!

It has ac. But yes its mainly on defrost with only a little air coming out of the vents because of the flap isn't all the way closed. (the flap is free moving too) This may sound dumb but where would I start looking for a leak? Does the vacuum come from the brake booster or where does it get its pull from? I know that its connected to the controls unit because i have taken the dash out and installed a used but newer unit into it... thanks for the response guys!!