It is basically a catch tank not a resivore.. tell me how the coolant can make it back into the radiator from the resivore? Does it lift the seal on the cap to put it back in? The coolant expands once heated, lifts the seal on the cap and pushes it out the overflow until it get to level at operating temp. There is no way for the coolant to get back to the radiator unless it can lift the seal on the cap and push it back in, and the cap doesn't work that way.. jmo