If your new separate master cylinder is equipped with two brake light switches, or if there are options to install them, this second switch is intended for the brake tester on the dashboard.this indicates that there is a difference in pressure between the front and rear brake circuits, signalling a problem in braking operation.
This light is often missing, even though the master cylinder is prepared for it. In this case, you'll need to plug the remaining hole, which can be done by screwing on an additional brake light switch, but it's much nicer to use the blind plug mentioned here.
This plug is suitable for sealing all the connections on your master cylinder, not just the connections for the brake light switch. This means you can also use it to close off part of the brake circuit, which is common on Trikes, as only the rear brake is controlled by the master cylinder.
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