Common problem. You need to get some fluid into the system, before it will bleed, which make's it bit of a 'chicken & egg' scenario.
As has already been suggested, you can push it back up from the calipers via the bleed nipple & a syringe. If using this method, it may be worth cracking open the banjo at the master cylinder until the fluid arrives. I have had success using this method, but it will almost invariably be a messy process.
I have had better results 'sucking' (no Jimmock!) fluid down to the calipers, again via the bleed nipples. To do this, you could use a largish syringe & a piece of hose, or invest in a 'proper bit of kit'.
I've had one of these for several years & found it to be an excellent piece of kit;
http://compare.ebay.co.uk/like/30065108 ... s&var=sbar
This would probably work equally as well, for a fraction of the price, just not as easy/clean to use as the Mityvac.
http://www.ebay.co.uk/itm/Reverse-Brake ... 796wt_1110
One last piece of advice from me, is whatever you decide to do , either push or suck (Down Jimmock). Would be to coat the nipple threads in grease (pref red rubber) this will help stop air getting draw down past the threads when the nipple is open, it will also help reduce fluid coming past the threads of an open nipple, should you opt for the push method.
Good luck & don't forget, brake fluid is EVIL
Almost forgot to say, the above methods will only fill the system with fluid, you will still have to bleed them as normal to get a firm brake.