If you keep applying the service brakes to control your speed on a steep downgrade, the brakes may fade (become less effective). Instead, shift into a lower gear to slow your vehicle. Use a gear that is lower than the gear you would use to go up that hill. Also use your retarder unless you’re on a slippery road (which could increase the chance of a skid). Don’t try to downshift after your speed has already built up. You may not be able to shift into a lower gear at that point.