One problem related to brake differential proportional valve has been reported for the 1999 GMC Sierra. The most recently reported issues are listed below.
On four separate occasions my 1999 GMC Sierra had problems braking cause close collisions. When pressing on brake at low speeds to come to a stop brake pedal was hard to press down. Seemed like brake booster went out. Had to apply extream amount of pressure on the brake to avoid collision. Had vehicle checked out by certified mechanic which found nothing after hooking up diagnostics test and also manually checking out brakes. Found nothing on computer to say why brakes failed. If anyone knows why this is please email me at krayztroy@aol. Com luckly there has been no collisions.