my diagram shows the brake warning lamp fed directly from the voltmeter positive, white wire, from the lamp is white\black where it tees off, one to the brake balance switch then to earth, the other goes to the oil warning lamp. Out of the oil warning lamp is a white\brown wire to the oil pressure switch.
It seems from this diagram (2.5 P.. Mk 2 , L H steering.) that the brake balance warning and oil pressure warning lamps are in series? Possibly as a lamp check for the brake warning?
For information, white circuits are ignition fed unfused, green circuits are ignition fed fused and purple circuits are constantly live.
PS, This is from an early ST manual, I have a later one in my workshop should none of the above make sense.