The max PSI on most the the Firestone airbags is 100 PSI, and most of the airbags that go on the 3/4 and 1 ton trucks will support a maximum of 5000# - thus according to Firestone - each 10 PSI will support about 500#. However, the objective of the airbags is to support only a portion of the hitch pin weight, thus allowing the truck suspension (springs and shocks) to work properly without becoming overloaded.
My 2003 2500HD rear suspension will support approximately 2000# hitch pin weight before it begins to squat lower in the rear than the front. Since the typical towing pin weight of my trailer is 2400#, I typically put approximately 8 PSI (to support about 400#) in my airbags. This allows the TV to ride level with the extra weight.
Since the pin weight varies with how many goodies I'm carrying each trip, I simply take a quick measurement from my rear bumper, and add air to the airbags until the bumper sits at its level height.
A quick check by looking into the rear wheel wells of the truck should show that the truck rear is sitting on the helper springs. Too much air in the airbags will cause the bags to support too much weight and allow the truck rear to lift off those springs - much the way the rear will ride when unloaded. This will allow too much up and down motion of the truck rear - and trailer front end - before the truck springs start working to control the ride. The resulting up and down and side to side swaying can become dangerous.
Been there, done that. Hope to not do that again.
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