Fill the master cylinder with fresh fluid, open all the bleeder valves a little, and let the system "gravity bleed". Once most of the air in the lines and wheel cylinders is displaced that way, top the MC off and try bleeding the brakes. You'll either end up with good brakes, or you'll discover the source of the fluid loss. At least try to do a little diagnosis before you start throwing a truckload of parts at the job.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!