1998 Ford Explorer Brake fluid resovior emptied but no visi

My brakes suddenly when to he floor. I checked the brakefluid and it was near empty. I can't find a visible leak anywhere yet a great deal o fluid went somewhere. The brake fluid was checked as full just a few days before this. Is it likely that the fluid leaded into the vacume assist unit via a blown seal in the rear of the master cylinder? If so, could the fliud damage the vacume assist unit?

Thanks!