My Benz has trouble when trying to back up. It does a fairly good job when the car has been driven for a while. When the car is cold, you might as well forget about it. Usually you have to get the RPMs up a ways before it starts backing up, but once it seems to sorta lock into reverse, it seems pretty solid and you can reduce the RPMs down to an idle. Possible vacuum problem...but where do I start looking for this? Thanks...