I use a syringe with brake fluid in and gently force it in through an open bleed nipple. Ensure that the other nipple is firmly closed and I put a bit of PTFE tape to stop the fluid tracking out the loose bleed nipple threads. Once one line is done ie, clear fluid appears in the master cylinder, close the nipple and carry out the same process on the other. Don't force it in too harshly as it can squirt out the master cylinder and as ever protect your paint work. As I say it always works for me.
