An effort was made to evaluate accurately truck air brake performance balance at low application pressures by means of comparing drum temperatures. This procedure produced excellent results, and is a fast, practical method for evaluating brake performance of combination vehicles. The drum, hub, and wheel assemblies used in this evaluation were all the same type and weight. The s-cam used was a 16-1/2 X 7 in. and the wedge was 15 X 7 in. Mixed combinations of these types were tested.
It was found that s-cam brakes are more effective when used on drive axles than when used on trailing axles. A tractor protection valve with a built-in pressure increase of 33% to the trailer brakes was installed to compensate for the more effective drive axle brakes and this provided the desired braking balance.