Why does every ride of a few hours my coolant leaks out of the over flow tube...

Why does every ride of a few hours my coolant leaks out of the over flow tube that goes to the thermostat? I road last weekend for 6 hours (not steady) and I just had to add 15oz of water to the radiator?

1 comments
  • Head gasket going bad perhaps. Sometimes they go and it leaks out the overflow and other times they leak into the oil and it burns while running.