I installed Airlift 1000 air bags in the rear springs on my 2003 Sienna.
A link to the first in a series of pictures......that show the installation process.
This does NOT increase the loading capacity of your vehicle.....
The intent of the Airlift 1000 is to maintain the original ride height when the vehicle is loaded.
They do work very well for this.
You add air when you increase the load......and let air out when you don't have a load.
Minimum air pressure to be kept in them is 5 pounds.
Max air pressure to be put in them is 35 pounds.
I use a bicycle pump to adjust the air pressure in them......only takes a few pumps to add air needed.