Why is brake master cylinder empty?

[fightingcec246]

I just returned from a extended stay way(12months) from home and had my truck in storage. Everything was fine with it, except when I checked my OEM master cylinder resevoir level - it was bone dry. I do not see any leaking around my brake lines to the drums. I am surprised as to this - I filled the resevoir back up, bled the brakes and "all is well" - or so I think. Is there anything I should be concerned for? It is puzzling that my resevoir was empty after the vehicle being in storage for 12 months. Is this normal? Or do I need to do more investigation and determine the culprit to the fluid loss. Thanks in advance.