Correct answer
Use the retarder and supplement it with the service brake when needed.
Use the retarder to help maintain suitable speed and the service brake when needed. This reduces friction-brake demand and heat without replacing their vehicle-stopping function. Select suitable speed and gear before descending and observe system limitations, especially grip and operating conditions. Not all lorries have an electric retarder.
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Keep the service brake continuously applied and disregard the retarder.
Rely solely on the retarder even if it cannot maintain safe speed.
Use the retarder and supplement it with the service brake when needed.
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