This is something I really struggled with too. I wound up disconnecting the line from the master cylinder. Having an assistant hold it in a cup of brake fluid. i put my vacume bleeder on the bleeder on the caliper and pulled fluid throught the line till it was solid full. then i fill the reservoir and hold my finger over the hole where the line goes and pump it till it starts to build pressure. once it is building pressure start letting a little through till you are getting no air anymore. then as fast as you can put the line out of the cup and hook it up. you would think that master cyl is a little pump and it would just fill the sys. but mine sure wouldn't. seemd like after i got it primed i had 2 good full pumps and it would lose it's prime agiain so i wound up doing it this way. long but i hope it helps to avoid the frustration i expierenced pumping that master cyl for hours on a saturday morning and geting nowhere.