In the trinitarian view of God as presented in scripture, the Holy Spirit has a unique role. The Holy Spirit fulfills a role that can be compared to the complementarian view of marriage that scripture presents in Ephesians 5.
The Spirit has equality with God the Father and God the Son and is clearly deity. He is not an outpouring of deity but is in fact God just as much as Jesus Christ is God. This is seen in scripture where the same roles and activities that the Son of God is seen doing the Holy Spirit also does. This is equating the workings of the Son of God and the Spirit of God to be both very similar. So by this line of reasoning to accept the Son of God as fully God, you also must scripturally accept the Holy Spirit of God as fully God.
The Holy Spirit, though equal to the Father and the Son, fulfills different functions within His role in the trinity. Though the Spirit is seen working in different ways than God the Father and God the Son, that does not mean that He is any less than God but is fulfilling a different role in the story of redemption in the plan of the trinitarian God.