1999 Sterling

Hydraulic or air? Have they been inspected? Have they been properly adjusted?

 

Have this checked by a mechanic. To me it sounds like brake foot valve is malfunctioning. For some reason it's sending full pressure to the brake chambers causing immediate lockup. Another train of thought is when you apply brakes, it sends pressure into chambers, then when you let off brake pedal it exhausts that pressure to atmosphere. The possibility exists that it's not exhausting that air pressure correctly holding it in the system which means even after you let off the brake pedal, that pressure is still there and not releasing brakes quickly but rather slowly. The valve causing that is your service brake valve located on a cross-member in the back near your drive axles. If the air can't get out, or get out fast enough then brake application is still active even after your foot left the brake pedal. When it's functioning correctly, as fast as your foot comes off that pedal, the air inside the chambers releases fully and immediately through that service valve in the back of your frame rails attached to the cross-member.