radiator not replenshing from reservoir

[tetsuo]

My radiator fluid goes low but it doesn't suck any fluid from the reservoir. What would cause this?

Thanks!

[94_240sx]

Radiator cap seal should open and close when there's a pressure. Take a look at it. You will see a rubber flap on the cap. I'd try a new cap and see. Make sure the hose is not clogged.