water valve purpose?

Heck, on most my cars/ trucks I install a brass 1/4 turn ball valve to shut the thing completely out in the summer. It's the only way to insure the vent system does not pick up residual heat from leakage, as factory water valves are NOTORIOUSLY leaky (throughput, not outside the system)

A 3/8" pipe ball valve and a long 3/8 nipple sawed in half makes perfect size for 5/8 heater hose.