We have an older house but all the water supply lines are copper. We have had a water pressure issue for a while now. It is the typical, in the shower and someone flushes the toilet, the pressure really drops. I didn't think that copper pipes would corrode inside reducing the water flow path. Is there a minimum pressure I should see at the input of the water meter? Should I call my water department and have them measure it or do I need to call a plumber?