So it is a DC injection brake, in which case disconnecting the link as I suggested will stop the brake from functioning, although I think you'll also want to disconnect the wire going to the Sp terminal on the braking module, as going by the diagram for it, the Sp appears to be some kind of detection signal (it only gets powered when the contactor is active).

However, I still suspect the real problem is the supply voltage sagging with the starting load.
What happens to the main phases powering the control circuit (L2 & 3 on the diagram) when everything is starting?