heater control

[s79vette]

My temperature control on dash heater isn't working properly. On my
Corvette, a few years ago, I added a small ball valve to close off heater
hose so now water deadheads instead of passing thru vacuum valve. My
question is: has anyone had this problem and did you replace stock valve
or go the add valve route?It causes the a/c to not blow cold air, because
of leaky valve. Steve