RE: What levels the rear bags?
TIRES!!!!
I had my '94 aligned countless times, and there was nothing else wrong in the suspension. I finally sold it, bought a '97, and after awhile, went through the same stuff. Shortly after i bought the '97, I bought new tires for it, it was fine, i drove it for a couple years and it eventually would wander all over no matter what. I fixed some supsension components here and there, but i knew nothing was wrong based on my inspections as well as inspections from others. I ended up swapping rims with a friend because i was taking a trip and my tires were bald, and i didn't want to buy new ones yet. As soon as we swapped, from my bald generals to his cracked good years, the car went straight as an arrow. No more wandering, pulling, none of that. Straight as an arrow. He put my tires on his car and started complaining right away. All over the road. He had a crappy suspension, needed wheel bearings, upper ball joints, and some other stuff, but on his tires, his car went straight, on mine, it wandered. My suspension was tight, but it was the same story. On my tires it sucked, on his tires it was great.
Also, some shops do better alignments than others. Steer clear of sears.
The rear bags operate together at all times. The system senses the height of both fronts and adjusts them individually to get the car level side to side, and at the right height, then it opens both rear bags at the same time and adjusts it until the left rear wheel is at the right height. With both bags open, the pressure will equalize naturally between them, so if you put the right rear on a curb, the pressure goes up, and when the system tries to lower the rear (even with the right rear up the left rear will be high, so the system will open both rears and the vent solenoid until the left rear is at the proper level) the extra pressure from the side on the curb will force air out and into the other bag or out the vent solenoid, until the pressure between the bags is equalized or the solenoids are closed.
Basically the car will adjust the height so that the sensors at both fronts and the left rear read correctly. When it adjusts the rear, both bags are adjusted at once.
I could throw some more engineering babble at you as to why it's good to do it that way (corner weights, etc) but you probably don't care. If your car isn't level side to side, adjust a front sensor. If that doesn't do it, make sure the solenoids are all functioning.
