I have a 1970 396 with A/C I have a new heater valve installed on the heater hose when the A/C is on MAX it is supposed to stop the coolant from going thru the heater core however it is not working properly the coolant is still going thru? When the MAX A/C is on is there supposed to vacuum to this valve to close it? or does the vacuum keep it open? thanks, Marty