Harbor Freight Pipe Threader no replacement dies

Our hot water circulator started buzzing and causing noise in my pipes. I plan on swapping it with the same make and model so I just have to drop the new one in. I saw the instructions stated “Air should be vented from the system by means of an air vent located at a high point in the system.” Exactly how do I go about doing this? Is this as simple as opening a tap? I have two 50 gallon hot water heaters connected to each other in my attic and the circulator is connected to a pipe coming from the second water heater’s drain. After doing some reading, I was concerned with airlock after installing a new circulator. I do have shutoff valves directly on either side of the pump. Thanks in advance for any advice. Below is a link to the circulator I have. https://www.supplyhouse.com/Bell-Gossett-103252LF-1-25-HP-NBF-22-Bronze-Circulator-Pump-Lead-Free

Good Day Everyone! This is my first post. Thank you in advance for your time. Here is my situation: This past winter the cold water pipe leading to the cold water faucet in our bathroom froze. After unfreezing the pipe, the faucet immediately started acting "strangely" [list] [*]The water would not go on when turning faucet (BTW, we have one of those two handle faucets - a hot side and a cold side) to the on position fully the water would NOT flow. But if I turn to the middle pressure (not fully on), the water would flow. [*]Over a few days, that resolved and a new version of the problem arose: [list] [*]The water (again, on the cold side only) would not flow regardless of handle position. [*]I did discover that if I ran the hot water for 5 or 10 sec than turned on the cold side (with hot still flowing), the cold water side would flow and it would remain on when turning off the hot side [*]For a few minutes after turning both off, I am able to turn on both sides freely [*]After 30 min or so, the problem would return. To get the cold to flow I would need to first turn on the hot [/list] [*]I looked up information online and figured I was dealing with an airlock. I tried running the hot side while placing a wash cloth to "cap" the water flow in effort to "break the airlock" - I would be able to get the cold to flow for a while but the problem would return after 30 or so minutes. [/list] Do you all believe this is an airlock? If so, what are the best ways to fix? If it is not an airlock, what do you think it is? Please let me know if I can provide any additional information. Thank you again!