The first time I followed the instructions in the Walter maintenance manual. It didn't work. Install the bleeder lines to the bottom valves and into a bucket or can. Turn on the boost pump. When the fuel is clear of air, leave the pump on, unscrew the lines. Install them in the top valves. Turn the pump off after removing the lines from the top valves. Walter's procedure allowed air to get back into the FCU when changing the lines. Regards. Dave Link.