my old ford did this once when i was off-roading,
the rear brake line ran on top of the rear axle and the exhaust brackets(plumber strap) broke.
long story short... the exhaust bounced up and down on the line until it crimped it and i totally lost my rear brakes.

i ended up getting a pre-bent brake line, and used stainless zip ties to attached it to the original on the rear axle