I have a 2001 tacoma 3.4L ABS 4WD. I noticed that the truck was making a hissing noise in the cab and verified that the brake booster was leaking by doing the brake pedal check once the truck was turned off. I took it off and had it rebuilt. I just received it back and put a mighty vac on it to verify that it holds vacuum and it does not. It is sucking air where the master cylinder plugs into the vacuum booster. This to me does not sound right as I was pretty sure that they are supposed to be sealed units. However there is a paper gasket that goes between the master and the booster. But if you look carefully there is a relief cut on the underside of the master and also on the booster where it receives the master. So I am pretty sure that the seal is not supposed to seal the vacuum. Any thoughts on this subject would be great before I put this thing back in.