yes if theres no fluid in m/cyl it has to be somewhere, if when its all filled up and bled nicely you still have no brakes it may be the m/cyl itself bypassing the seals when there is resistance in the line ie your shoes contacting the drums, normally when this happens the brakes will bleed ok still as with the bleed nipple open there is sweet fkall resistance in the lines so m/cyl will work normal but as soon as you close it itl bypass
may not be your problem but id check where your fluid has gone, also if you drained the lines and left empty moisture may have gotten into lines and rusted them from the inside out?