The NACCO axle uses the same discs for both axle ends. I think maybe when the discs hang up you get a jolt at the drive hubs. You can check to see if the booster is holding the master cylinder under pressure. pump the brakes a couple of times and then loosen the bleeder. If the master cylinder has pressure on it you will see a lot of fluid coming out of the bleeder. The master cylinder will need replaced or kitted due to the wrong fluid.

Thanks, kinda thought I might have to do that. Would this issue cause the unit to go in reverse while braking though? Its weird, while going forward and hitting the brakes at the last second it feels like and looks like the front wheels go backwards for a second. Could I also have an internal issue with the booster or something else?

You need to pull the brakes apart and change all the seals. I had this happen to me. The discs also will be damaged if the brake fluid got past the seals.

Any ideas? I've since found that someone put regular brake fluid in instead of dexron III. Flushed it out but still having issues with brake chatter and hard pedal. Adjusted the pedal and bled the brakes a few times. Wondering if there's an issue with the booster. Any help would be appreciated. Thanks.
Also just noticed that when going forward and hitting the brakes to stop, just before the unit comes to a complete stop the drive wheels seem to try and go in reverse for a second...don't think this is normal but figured I'd throw it out there.