The emrgency brake should hold regardlessof the level of the ground, so that will have to be checked. More than likely it is the transmission. To stop it from rolling back in the meantime, when you park it, park it close to the curb, and turn the wheel all the way towards the curb as far as it will go, the curb will be used as a stop. make sure you get very close to the curb, cause with that size car if you give it too much room it could go over the curb. I don't have to tell you this is a dangerous situation and should be fixed as soon as possible to avoid an incident. Hope this helped.
Lisa
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