Since the brake resorvoir is also the clutches did you go low on brake fluid shortly before this issue and add fluid to the resorvoir? Might explain the lines getting air.If you also have a fluid lose at either a brake line or more commonly a worn out caliper this could seep air into the clutch sytem via the brake resorvior,thats the only way if theirs no culpreit along the clutch system and providing the clutch was bleed properly .I would start by checking all the brakes and lines for wetness if the clutch slave / master cyl and lines look dry.I dont remember the trick off hand to bleeding the air out of these systems but I did learn the process on this forum .