air won't stop blowing out of floor vent

[1972BlueC20]

Hi all,

Need some help, I have a 72 C20 with aftermarket AC, so it's technically a NON AC truck originally. I have a busted heater core which I'm going to be replacing this weekend, and I've looped the hose from the intake to water pump to bypass the heater core for now.

I noticed today for the first time on the way to work since it's a little cooler outside that even with the ac and the heater ect completely off, I'm getting a constant flow of air coming through the floor vent and a little I think through the vents in the dash too....I'm assuming this is from air coming into the engine bay from driving down the road.


How can I stop this???

Any help or suggestions are greatly appreciated.


-Brian