Experience
Had it happen on an A340 Green system, both 1 and 4 pumps caused the hydraulic fluid to turn to black sludge...ended up changing main delivery lines and flush the system over and over....what caused it? Never did find out but suspected air in the system...airbus are very difficult to bleed all the air from the hydraulic systems...you end up bleeding on the pressure/case/supply lines on the pumps..