There's a small hole in the leveling valve arm. Loose the nuts for the leveling valve mount, take a Golf tee, stick it in that hole. There's a matching hole on the leveling valve itself... when that tee lines up with that hole, you've got it calibrated. Leave the golf tee in place and secure the leveling valve.
there is no way to adjust the ride height other then aftermarket suspension parts
No you can not, at least not with the original or OEM suspension.
Peterbilt made (and continues to make) a number of different models available for a number of different purposes. They won't all have the same suspension. Construction vehicles would have the option of Camel Back, Hendrickson Walking Beam, or Chalmers Suspensions. Road tractors would most likely have a four bag air ride suspension.
I am pretty sure the dealer can do it or you can but a NGS scan tool or something similar.
Sounds like you have a leak in the air suspension. Check out if it does or it will burn out the pump. Monroe makes a replacement set up for the " air -Ride". It runs you around a grand to have it replace the OEM design.
There's a suspension valve towards the rear of the frame... there's a little room to tilt it so that you can raise or lower the air suspension. Just loosen the nuts on the back of it, tilt it, tighten it down when it's where you want it to be.
There are two bolts holding the leveling valve in place, with the one on the bottom being more oval shaped...rotate the leveling valve counterclockwise to raise it, and clockwise to lower it. To calibrate it, you'll notice a small hole in the leveling valve arm, behind the hole for the bolt. You take a golf tee, stick it in there, and adjust it until it lines up with the matching hole in the leveling valve itself.
The Torsion bar is a suspension component. It essentially "twists" ever so slightly to adjust ride height. This is adjusted by a special tool specific to the manufacturer.
Low air suspension refers to the fifth wheel height from the ground. The suspension system (low air leaf) is a suspension system with a lower ride height and is used by many transportation companies because it allows their truck to hook up to a variety of trailers. In modern days, low air leaf has been replaced with Flex Air suspension (low air leaf is still available). Flex air has the same ride height but weighs several hundred pounds less, thus allowing the truck to haul more weight in the trailer.
One can find an air ride suspension specialist in San Diego on craigslist. AOS is a craigslist user who offers support as an air ride suspension specialist in San Diego.
Ebay
There's a leveling valve at the back, and, if you loosen the four bolts, you'll find that you can rotate it clockwise or counterclockwise as needed, then tighten it back down.