Low Water Pressure

Checked a few things out. It looks like there are no filters/ strainers other than the check valve right at the inlet. Now there still is the large incoming filter housing that is still in the picture that I run without a filter in it. I can pretty much see everything. What should my incoming pressure be? I am going back down this weekend and will remove the pressure regulator. I had done this before and it did not seem to make a difference.
Flushing seems like a good idea. How about bleach? How do you go about flushing or should I just replace the fixtures? The shower is the only one that really bothers me.