please help

hi i have a 93 mod toyota landcruiser 80 series ,petrol 4.5ltr. and my radiator won't fill up after it cools down, i drive 40-80kms and than over night my radiator fluid is about 40mm from top of radiator ,its not loosing fluid at all just a vacum prob. i have replaced radiator,hoses,resoiur tank,tried half a dozen rad caps and still no differnce. i am affraid it will overheat .please help because i rely on my cruiser and cant afford to go to mechanics.cheers and look forward to your reply.