how much will an impeller pump??

I have had a problem with my 77 Merc 4.5 as it quit pumping water. I just recently replaced the impeller and when I put it back together it still wasn't working. After submitting a post I was given some other advice for things to look for. I pulled the engine off the top of the entire leg and checked all the places where the water will be circulating and didn't find any blockages. The gasket on the bottom of the engine was it pretty rough shape. I am in the process of getting a new one. I am not sure if this was my problem or not. I have 2 questions. Once I get the gasket and put it back together should the hoses where the water would get pumped through be air tight. Meaning that if I tried to blow air into the pee hole(no jokes please), and put my finger over the end of the line near the bottom of the leg where it would connect to the water pump... should it be air tight? Also Laddies had a great suggestion while I have the Lower Unit off, about putting a drill on the end of the drive shaft to test if my impeller is working properly . I tried this, and I can tell that the impeller is turning, but I am not sure how successful mytest is. I can see a little water pumping out the top of the water pump, but it is not like it is shooting out or anything. Using an average drill, how much water should I see flowing through the top of my water pump?<br /><br />Thanks