brakes on 94 gmc

brakes sometimes go to floor have to pump to have a brake, brake light comes on. someone told me the black box on the brake system was bad. no clue. bristol

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No clue?, this tells me you need professional to help , many conditions in the brake system can cause this, no accurate diagnosis can be done remotely..

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? is your master cylinder full of fluid. ?sit in one spot and pump the brake pedel 20 times ?do you see a puddle of fluid ? if no than does the brake pedel go to the floor if it does you need a master cylinder
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