A truck driver has asked to begin serving prison time before he is sentenced as he struggles to come to terms with driving a 62-tonne vehicle with faulty brakes that crashed into a school bus and seriously injured scores of students.

Brett Russell, an experienced truckie who held a heavy vehicle licence for 20 years, knew the engine brakes on his truck and trailer brakes were defective but decided to drive anyway.

He knew if he had to stop suddenly for an emergency he was "in big trouble", his barrister Richard Edney told Victoria's County Court on Friday.

"As soon as I'd seen the sign 'road work ahead', I had absolutely no brakes, you're 62-tonne and you're going down a hill and you run out of air - nothing is going to stop that truck," Russell told police during his interview.

"Once

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