Method of measuring the weight of bulk liquid material within a container of a tractor-trailer combination (1) in which air bag (16) pressure of a pneumatic suspension system is sensed (60) and correlated with the weight of the bulk liquid material within the container. In accordance with the method, the tractor engine (36) is turned off and supplemental pressurized air is introduced into a pneumatic circuit (2) feeding compressed air to the air bags (16) and while not applying the brakes of the trailer.