I just bought a used Spektrum DX3.0 with receiver. This was installed and hooked up to my 4wd slash. I managed to get everything working except brakes while going forward. The brakes work great while going in reverse but not in forward. I have been going through the manual but can not locate anything about this. Any help would be greatly appreciated.

I dont believe the Traxxas ESC has any settings to remove brakes, so it could be that your transmitter settings are not correct. Are you familiar with the epa, trim and servo reversing functions of your DX3.0? I would make certain that these are properly set and try to re-calibrate the radio to the ESC.