We use an all american 921 with no problems, had it for 6 or 7 years.
It sounds like you are not leaking at the lid seal but where the little weight sits on the spicket. Ours spits a little steam out there and the weight kind of rattles back and forth. The little round weight has 3 sides to it one for 5, 10 and 15 lbs.
Assuming your system is not faulty, I would guess you have the heat too high if you are getting excess steam around the weight. If you boil water steam is created. If the steam cant escape pressure builds. At 10 lbs the weight is designed to let the excess steam out of the canner to keep it at 10 lbs. If your heat is to high you could be boiling your water off while the weight maintains the 10lbs. Find the minimum heat that will maintaining 10lbs and you shouldnt loose so much water.
Or, maybe your not starting with enough water in the canner to start with.

Your gonna lose some water in the process.
Just a couple ideas for you.