Can air in lines cause low water pressure?

Yes and No if you are talking about a side outlet pressure relief valve on a tank water heater then No. If you are talking about a top end out let of a water heater then maybe because the air over time could dry out the rubber seal and bleed out and make it slowly leak which in my own thought is very unlikely because most air in water lines seem to absorb into the water, like most air hammer resistors that don't have a bladder in them fail to work over time. One theory is that lets say you fixed a leak on a water line going to your water heater and now when you turn on the pressure the air goes into your water heater tank and stays at the top of the tank and since air can go threw smaller holes then water can it bleeds out your PRV and some how opens holes on the seal bigger to allow water to bleed threw so depending on what PRV and where it is located and how old the PRV makes it a yes or no answer.