If the failure is due to leaking break lines, pump the pedal a few times to build some pressure. In the meanwhile, shift to lower gears to get engine braking, and try to use just enough hand brake that the wheels don't lock up.
In extreme cases, you might want to get the side of the car to touch the divider/ hill/ whatever is available to get the car to slow down. This is only as a last resort. Thats what my father did once in his 77 Corolla.