That plunger senses pressure differential and moves the plunger in respect to the difference. If you have a total brake pressure loss the plunger would go fully into a failure position and that turns on your red BRAKE dash warning light. If it gets partially tripped it would limit brake fluid flow and you would see very little fluid volume when bleeding the brakes.. I always gravity bleed after a test drive, take off the master cylinder cap and put a section of vacuum line on the bleeder when it is open. The vacuum line keeps the fluid from chewing paint and you can watch the fluid to see if there is any air in the fluid..
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1971 Boss 351 , 17,000 miles .. 1971 Buick GS455 .. 2003 SVT Cobra .. 1971 GTO .. 1969 Camaro Fathom Green .. 1969 Camaro Hugger Orange .. 1970 Buick GS455 .. 1970 BUICK GS350 .. 1998 SVT Cobra 1900 miles;. 1973 STAGE 1 GS . 1971 Mach 1 351C 4 speed , first car. 1970 Mach 1 428 SCJ . 1970 SS 350 Camaro . 1971 442 ... 1968 Buick GS400 . 2015 Lexus IS 350 F Sport 2021 Badlands Bronco Sport
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