This is an A body 4 wheel drum distribution block.
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These have no proportioning to them. They serve as a junction block for all of the lines to attach and it includes the federally mandated warning light switch.
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The threaded stud is for the warning light switch. It is a simple barbell shaped piece that slides up and down in a slot in the block. In theory, it is supposed to block off flow to either the front or the rear if there is a leak. I've never known them to work that way though. The advice that I've recently heard is to remove the "barbell" .....this turns the block into a true open distribution block where the system has no restrictions.
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