On level ground with the engine running, the machine should not move even if there were NO brakes on the machine.
If it moves at all then that is evidence that the drive pump swashplate is not centered when in neutral.
It sounds as though the drive pump is out of null and the pump always being on stroke has finally wore the brakes out.
also: If the machine stops moving after a few seconds, oil could be trapped in the orifice at the brake/2 speed manifold, not letting the brake pressure evacuate quickly enough. The pump would still need to be nulled.
If the machine keeps moving, disconnect the 4-pin plug at the drive pump to verify the EDC is not getting voltage applied.
If it keeps moving with the plug removed, the pump definitely needs nulling.
If it stops moving with the plug disconnected then the drive circuit is not shutting off when in neutral.
This could leave the brake release circuit activated as well.
You could try disconnecting the coil wire (white with red stripe) on the brake valve.
Try driving the machine (being careful and on level ground) with this wire disconnected. If it drives easily then there is a brake circuit problem.