Unloaded, or at max gross weight? Rear brakes on big rigs are designed to stop a loaded truck and the driver is supposed to be aware of the tendency for the rear brakes of an unloaded truck to lock up and drive accordingly. The proportioning valve needs to be installed between the driver's ears.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!