Why didn't they keep the brakes on when revving the Delorean to 88mph?

The car had to reach a velocity of 88 miles per hour. It had nothing to do with the speedometer hitting 88 miles per hour.

In the animated series episode Gone Fishin` Jules Brown (one of Doc Brown's sons) places a spare flux capacitor in a barrel. When the barrel falls over the waterfall and reaches 88 mph, the flux capacitor activites and moves Jules Brown through time (and space) before crashing at the bottom.