When driving down a long and/or steep downgrade you should:

The use of brakes on a long and/or steep downgrade is only a supplement to the braking effect of the engine. Once the vehicle is in the proper low gear, the following are the proper braking techniques:
• Apply the brakes just hard enough to feel a definite slowdown.
• When your speed has been reduced to approximately five mph below your “safe” speed, release the brakes. (This brake application should last for about three seconds.)
• When your speed has increased to your “safe” speed, repeat steps 1 and 2. [Commercial Driver License Manual]