The bleed-off (or bleed or bleeder) screw is used to let the air escape from the system as you fill it up with antifreeze/distilled h2o. You just unscrew a little ways (you don't have to remove it compeltely, fill the radiator expansion tank with fluid to the correct mark, and then tighten the bleed screw back down. Drive the car a little, let the car cool down and repeat the fill-up bleed-off process a couple to a few times until the system runs at the proper level and air isn't trapped in the system anymore. Then the heater works correctly and you don't overheat the motor.
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