Coming in from left field here. If the repair is out of sight, have you considered JB WaterWeld epoxy? It can handle high pressure and up to 250F. I carry it in the travel trailer in case I need to fix something with the fresh water system. I got it to stick on a small crack in my water tank while it was still actively leaking and a year later, it's all good. You can fill the crack and sand it down as well.
https://www.jbweld.com/product/waterweld-epoxy-putty