I moved into a older house and built an addition onto it. I used that new Pex with a manifold. 3/4" copper going to the manifold, 1/2" pex going to each fixture. I have a well and a water tank, which I did not replace. My problem. Low water pressure. If someone is taking a shower in one bathroom forget using anything else. The pressure cuts out. I did not replace the existing tank, looks good, has 45 lbs in it. I believe the other owners said the pressure wasn't good.I didn't replace any old copper with Pex except the Kit. Could it be the tank? or maybe the 3/4" iron pipe going to the tank?
Hope you have an answer.
Thanks
John

What is your pump pressure switch set at? Sounds like a volume problem.