My '97 Century Class also "bleeds past".  The control valve is near the firewall in the engine compartment beneath/inside (meaning toward the center) the air filter.  I pulled one from a wrecked truck at a junk yard - it appears to have a small slot, allowing bleed by design, maybe designed to keep the AC core from freezing.

My Detroit has a shutoff valve for the driver/passenger heater and one for the sleeper heater.  In the fall and spring I usually lift the hood and turn in off mid morning and turn it on late evening.  My sleeper heater control is not working; the linkage appears to broken.