When I did mine I didn't look to see what size fit. Have to measure it and get one a mm or so smaller. I suspect in the 5-7mm range. I do know from experience though that bleeding these the conventional way is a serious PITA as you only have about 4 pumps before the reservoir needs refilling and to force air to the a bleeder that far down is nearly impossible in such short secession. It is far better to use a power or pressure bleeder or although it can be a bit messy,.. back-bleeding would be the preferred method for these.