IIRC there is a valve on the body with an arm that attaches to the suspension arm. When there is a load the arm activates the valve to provide more pressure in the system to lift the rear. If this arm link fell off you might have such an event. I think they sometimes also seize with rust.

Tom W

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.