Bleading new Brake Lines and Master Cylinder

[Strut]

I have ran new Stainless lines front to back and have a new CPP Master Cylinder and distribution block. Im using a Power Bleader to pressure test the system before I put fluid into it. I put 15psi to the system and over a 12 hour period it will lose about 6psi. I have checked all the fitting with soapy water and cant find and leaks.
Im wondering if this is exceptable or if I need to try and track down where Im losing this pressure? What do you guys think??

Thanks