Q. Why are the apostles “filled with the Holy Spirit” when they pray for boldness after Peter and John are released from prison, when they have just recently received the Spirit on Pentecost? Isn’t the receiving of the Holy Spirit a one-time thing as opposed to how it was in the Old Testament times? If there are deeper levels / experiences, what do they consist of?

As I understand it, on the day of Pentecost, it is the community of Jesus’ followers that is filled with the Holy Spirit, as the “new temple” of the new covenant.
Under the old covenant, when the tabernacle was first set up in the wilderness, “the glory of the Lord filled the tabernacle,” to such an extent that “Moses could not enter the tent of meeting because the cloud had settled on it, and the glory of the Lord filled the tabernacle.” Later, when Solomon built the first temple in Jerusalem and brought the ark of the covenant there, similarly “the cloud filled the temple of the Lord, and the priests could not perform their service because of the cloud, for the glory of the Lord filled his temple.”
Unfortunately the Israelites broke this covenant and they were conquered and exiled, and the first temple was destroyed. Shortly before this, as Ezekiel saw in one of his visions, “the glory of the God of Israel went up from above the cherubim [i.e. upon the ark], where it had been, and moved to the threshold of the temple.” As Ezekiel looked on, “the glory of the Lord departed from over the threshold of the temple” and it was escorted away out of the city by angelic beings. (I’m always horrified when I read about this departure of God’s glory and Spirit!)
There is no record in the Bible of God’s glory filling the second temple, which was rebuilt in various stages after the return from exile. I believe this is because, under the promised new covenant, the community of believers was to constitute the new temple. As Paul wrote to the Corinthians, addressing the community corporately, not the members individually, “Don’t you know that you yourselves are God’s temple and that God’s Spirit dwells in your midst?” Paul wrote similarly to the Ephesians:
“You are no longer foreigners and strangers, but fellow citizens with God’s people and also members of his household, built on the foundation of the apostles and prophets, with Christ Jesus himself as the chief cornerstone. In him the whole building is joined together and rises to become a holy temple in the Lord. And in him you too are being built together to become a dwelling in which God lives by his Spirit.”
So this is what I think was happening on the day of Pentecost. As I put it in the title of a sermon I preached one Pentecost Sunday when I was a pastor, it is the “filling of the new temple.” This is something different from the filling of an individual believer by the Holy Spirit. I believe that such an individual filling takes place in one sense on a one-time basis, but that in another sense it can happen on a repeated basis.
Everyone who becomes a committed follower of Jesus receives the Holy Spirit as a gift, to equip and empower them to serve and to live a holy life. So this one-time filling is not a matter of us getting more of the Holy Spirit. Rather, it’s a matter of the Holy Spirit getting more of us. Christians throughout the ages have reported an experience that sometimes goes by the name of “complete surrender,” in which they realize that Jesus must have unchallenged lordship in their lives. They therefore surrender their wills to do only God’s will. And many report that concurrently something happens that they call the “baptism in the Holy Spirit.” (“Baptism” is simply the Greek word for “to fill by immersing,” so this is just another way of saying “the filling of the Holy Spirit.”) The point is that the Holy Spirit, who has now been given free access to the whole being, fills all those areas in our lives that had once been closed off.
This experience is known by other names as well, such as the “Second Blessing,” but it does not have to come a long time after a person’s first commitment to Christ. Churches in the Pentecostal tradition associate it with receiving the “gift of languages” (which I discuss in this post), but the experience was well attested in church history long before the Pentecostal movement began in 1906. Basically it is a one-time “filling” with the Holy Spirit that occurs when we open our entire life for the Spirit to fill.
This is different from the kind of “filling” we might need and experience on a recurring basis when the Spirit makes use of us in a special way as an instrument of God’s work on earth. This recurring kind of filling is, as you say, described in the Old Testament, as well as in New Testament passages like the one in which the apostles pray for boldness. These are those situations in which, as I explain in this post, it is said that “the Spirit of Yahweh clothed herself with” a certain person, in effect “putting that person on” like a garment so that they could become God’s instrument.
Even when this particular language is not present, and we hear simply about the person being “filled with the Spirit,” the idea is the same. A special measure of God’s presence and empowerment is needed for a particular task, and so it is granted for that occasion.
I hope these distinctions are helpful in answering your question.
yes for wonderful the Holy spIrIt in altot the years with us in mankinds help and sure the truth with him to show that the word the truth and righteousness and i believe today we can be fild of the Holy Spirit many time and be more powerful to be used of God to his purpose in life for be upbuild the kingdom of GOD IN SIDE US DAILY ,THANKS AND BLESS,KEIJO SWEDEN
Thanks for answering!