I think Bubbles may be close with his suggestion in post #17. If you have a Shurflo pump you may have a "back pressure bypass valve" built in to the pump which is factory set to 55 psi and will open when the pump pressure goes to high which in return causes the pump to run constantly without ever shutting down. If you are able to somehow measure the system pressure and find that it is more than 45 psi you could try to adjust the pump pressure down to 40 psi using a small 5/64" allen key on a tiny allen head screw on the pump head. This procedure is not mentioned in the owner's manual, so be careful and don't do more than a 1/4 turn at a time. My pump is a Shurflo model #4008-101-A65, yours may or may not be the same.
However, large air pockets somewhere could still be a possibility but I doubt it.
2018 RAM 5500 Laramie CC
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