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I Have A Mate Here In Australia Who Has Just Bought A New 2008 Gmc 2500 Crew Cab Duramax On His First Trip Towing His Van Which Weighs Aprox 3 Tonne The Front Of The Truck Continously Bounces Up And Down He Has Air Bags Fitted To The Rear And Changing Pressures Makes No Difference Sounds Like Shockers Are They Prone To Being Faulty From Brand New
Any Ideas Much Appreciated
Sounds like excessive tongue weight. If by van you mean a conventional trailer he should have a tongue weight of between 10 and 15 %. On a six ton trailer that translates into 600 to 750 lbs. If this is exceeded the front of the truck will tend to get lighter and the back of the truck will get heavier even with an air bag system. Here is a good site that explains some of the physics.
http://www.etrailer.com/faq_weightdistribution.aspx
A weight distributing hitch will help as it transfers some of the weight back onto the front end reducing this imbalance. A light front end also affects steering and sway negatively.
Hope this helps.
X2 on that weight distributing hitch for heavy 'bumper pull' trailers.