When you lose the engine you also lose the vacuum created by the air rushing through the manifold that powers the brake booster. So if you want to keep your assisted brakes on the EV conversion, you need to add an electric vacuum pump.

You can pick these up cheap from AliExpress, or pick up a second hand Audi/VW unit as I did. Mine came from a 2003 VW Touareg via eBay and included its mounting bracket, which made mounting it easier: I just welded the existing bracket to the side of my adaptor plate.

These pumps just have a simple two wire connection and can be controlled by a simple limit switch. OpenInverter provides a board to do this switching based on the input from another standard VW part, a vacuum pressure sensor. Rather than make up my own unit (though I might do this in the future), I went for this option.