I have done some investigating for my self and have seemed to have found the problem. With drums there is a one way valve which is spring loaded and holds a certain amount of pressure in the break lines...(enough to hold the drum shoes out). With disc breaks you have a similar valve but with a small hole in the centre allowing no residual pressure. They are quite obvious when pulling apart a standard 1200 MC. I know there is a fair bit of fuss about upgrading master cylinders when upgrading brakes but could someone please explain why? I can understand the use of a booster but apart from that I can't see how it makes any difference.? There is the obvious variations in bore and stroke but shed some light....