If you are bleeding the brakes using the pedal pump method, the normal procedure is to unscrew the contact part of the switch from the brake failure sensor assembly. The pedal pump method creates a fair amount of brake fluid pressure which can cause the piston in the sensor to move and the switch to operate when you bleed one of the circuits (the equivalent of one of the brake lines splitting in use). When you close the bleeder nipple and restore pressure on both sides of the piston it may attempt to move back to its normal position jamming the little pin on the actual switch. The pin may break or you may find yourself with a permanent brake failure indication if the pin jams in the operated position. Bleeding uses a pressure bleeder avoids this problem because the actual fluid pressures in the system are quite low and should not cause the switch to operate.
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