I came home to a watery mess in second floor bathroom the other day. I located the leak at a coupler under the faucet. The inch galvanized supply line was threaded back in the day to fit 7/16 threads. I went to the local plumbing only to find out that 7/16 inch couplers and valves are not made anymore. A nice plumber in the store said that it would be best to take the steel pipe out and put in PVC. The main plumbing is copper but it is unknown how it goes from copper to the galvanized. The plumbing from the basement to the second floor runs through the living room wall but the pipes below the bathroom are above the old slat and plaster ceiling in the living room. Eventually I would like to replace all the plumbing in the house to PVC since every faucet and toilet have a mixture of copper, cast iron, old pvc, and galvanized steel. This leaky sink is the third plumbing problem to arise in this bathroom and is really starting to get to me. Any advice on how I can fix the leak?

Thanks and blessings,
Optimistic New Homeowner