brake master cylinder qestion

[wacks]

need help guy's, im converting my brake system into a 4 wheel disc system im using stock vw calipers on front and 914 calipers with emergency brakes on the rear. my friend told me that the standard master cylinder could not supply the pistons of the caliper i am about to use? is this true? do i need to replace my present dual master brake cylinder? if ever, what shold i use? thanks