I noticed my Master Cylinder wiring has two brown wires and only one is being used?

The switch at the MC has only one connection.

Why two brown leads?

I think my MC is a 19mm unit.

I will post a picture later.

The stock switch on the 17mm MC has two connections. The replacement switch on the 19mm has one one because the switch grounds through the body of the MC. The brown/white wire is the one that should be hooked up. The solid brown wire becomes extra and should be tucked out of the way where it won't contact the brown/white wire, which would make your warning light flash erroneously.

Thanks Mark. They both looked to be brown, but it was a bit dark under the car last night. Looks like it is all good.

If the solid brown wire is hooked to the MC switch, the switch will not function to warn you if the MC is failing. It's worth taking a little time to confirm.