well the other day i was backing my truch out of the garage and i noticed that the ABS light came on so i looked at my flud and looked for any leaks and pedal was hard and there were no sign's of anything being wrong but i drove it to school 3hrs and the light never came one and it only seems to come on when i back up. but if i am just to drive forward it does not come on any ideas what the problem would be?