The is the OEM GM brake booster vacuum port for the TBSS/NNBS intake manifold.  New GM intakes (including the one we offer) ship with the hyrdroboost block-off plug in place. If your vehicle has vacuum assisted brakes, this is the fitting you will need to install in place of the hydroboost plug on the intake.
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To install, simply depress the red collar on the existing fitting installed in the intake. While depressing the red collar with one hand, use the other hand to pull the fitting/plug away from the intake and it should slip right out. The new fitting can be installed by simply pushing it into place.
