Water backflow testing. Pulled the 8” back flow apart. 1st test was full of rust and the relief valve was only opened 1/2 way. Soaked the relief in ZEP calcium lime and rust remover. Checked the spring and cleaned 1st check. I also reassembled the device, installed 2” PVC pipe to drain and open shut off valve to blow out all rust and debris from the inlet pipe. Reassemble the device, found the relief rubber diaphragm is ripped and the relief is spraying. I need to rebuild this device. Test and certified both back flows per State Code.