car is over compounded. decrease the series field strength by taking out some of the turns in the series field. you want to keep the voltage at the generator armature below 2 volts after stopping. if it is near 6 volts as you say, it will only take a few more volts to start it pulling through the brake. you will see that the car speed in leveling is quite a bit faster at level speed in the down direction. keep adjusting the turns in the series field until you get up and down speed close to equal
