If a passenger coach experiences complete hydraulic/pneumatic service brake failure while descending a steep mountain grade, what emergency measures should the driver take?

1. Perform sequential downshifting (抢挡/减档) to harness maximum engine compression braking, look for an emergency runaway escape ramp (避险车道), utilize parking brakes progressively, and rub roadside embankments as a last resort
2. Turn off the engine ignition key and coast freely in neutral to look for flat ground
3. Instruct passengers to jump out of the moving bus doors one by one
4. Honk the horn and steer over the cliff edge into treetops