2000 Pontiac Montana replacing mastercylinder

Replaced my back brakes shoes cause pedal was spongy and had only front brakes. While fixin them I noticed wheel cylinders on both sides were leaking so I replaced both sides and both back brake lines. Brakes worked great untill two weeks went by but now all of a sudden brakes are spongy and brake resivore is almost empty and can't see any where it could be leaking