When the brakes on the prison rig are cut they leak hydraulic fluid making it unable to stop. A vehicle that of size and weight would definitely require air brakes which not only don't leak fluid but apply automatically if the air line is cut.
At the beginning of the film, Deadpool says he was on top of 2,000 gallons of gasoline. In fact, he laid on seven 55 gallon drums for a total of only 385 gallons, and several of the drums were labeled as kerosene. However Deadpool tends to exaggerate his statements and was just being dramatic.