Brake warning light on dash

What would cause the brake warning light come on when applying the brakes. This is a 69 Dart that has a sensor on the distribution block. Is it air in the lines? or too low of hyd pressure that this sensor detects? It was converted to 4 wheel disc brakes by SSBC and now not stoping good at all. Brake warning light comes on if you really push hard on the pedal to try and make it stop.