My 40 gallon water heater blew and I replaced it with a tankless unit and have been slowly upgrading the plumbing lines (so they don't stick though the fist floor ceiling) and added a pressure reducing valve (PRV) since then.
It took me a few weeks to get the tankless unit shipped and installed. During that time I had the hotline where the tank use to be open to air with no pressure on it and started to noticed some back-flow from the cold side into my hot pipes from some of the upstairs faucets, so I threw a push-connect ball valve on it. About a week later, the faucet in the laundry basin started to also started to backflow into the hot line (I could drain the line by opening the hot valve after using the cold). The supply pressure at the time was about 140 psi, and I just assumed it was due to the lack of pressure on the hot water side.
I was able to get my on-demand hot-water heater installed a few days before my PRV came in, so I only had it connected up to the part of the house that included the laundry basin faucet. When I fist connected it, the faucet leaked a heavy trickle. Alternating the hot and cold from fully on to fully off got the leak to stop.
After installing the PRV I added connected the upstairs sinks to the hot water line. However, once connected, both started to leak cold water. I tried alternating the hot and cold from full on to full off, but the leak got slightly worse this time.
Why are all my valves failing during this install? Isherwood's comment on this question lead me to believe the initial city pressure, or water hardness have something to do with my current issues.
Is there a trick to fixing leaks that appear after restoring line pressure?