I had a problem with DOT causing my master cylinder and front wheel cylinders to leak slightly (incompatibility of the rubber), I contacted Wagner Brakes on how to flush the system to remove it, they suggested using isopropyl alcohol to flush, then a thorough flush with DOT 3/4 to clean it out. They did say that a small amount of DOT 5 will not hurt anything as it is similar to the assembly lube used to put the whaeel cylinders and master cylinders together. After swapping back to DOT 4, I had no futher leakage issues.
