I have a 1985 Bryant central air Furnace model 395BAW048095 with a White Rogers 36E gas valve. The flue is metal and there is a fan within the furnace to create a vacume at the base of the flue pipe conection that turns on when a call for heat comes from the thermostat at startup to ignition. The spark fires but no gas is released by the valve for the pilot to lite. Added information: The blower fan is set to continually run, I replaced the pressure switch (306621) and the limit switch (HH12ZB180), and the circuit board burned out two years ago and I replaced it at that time with a Carrier HH84AA021, I replaced the blower motor three years ago. Any help would be appreciated.