71 c10 rear brakes not working

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I recently bought a 1971 c10 that sat for about 10 years. the previous owner took arround the block once every couple of months, so when I drove it the rear brakes where sticking , I put new shoes, turned the rotors and rebuilt the wheel cilynders. now I'm not getting any pressure to the rear , I bleeded the master cylinder and theres pressure there. theres another unit under the master cyl but the lines are preety stripped, so I couldnt loosen them. whats this unit and whats its purpose? thanks