This is a very simple pressure air tank for lego pneumatics. I thought of this after I built a lego air pump and saw how expensive lego pneumatic parts were. It's made of a sauce container I got at a chinese restaurant, a T pneumatic junction, and glue. It isn't tested and I have no idea if it will blow up under pressure or not, but I think it will work for lower pressures. I will get back to this Instructable after I test it.
Get a container. Bore a little hole in the cover of the container. Insert the T connector pin into the hole and glue around it. Glue along the cover where it will contact the container. Make sure it is sealed. There you go! I recommend that you add a little bit of extra glue so the cover can stay on tighter.