I have a manufactured home that had that gray stuff in it. At 10yrs old,right after I bought it we started springing leaks.I'd repair them,just to get another leak somewhere else,usually at the connections.Sometimes the lines themselves would split. Finally,to make a long story short,I started replacing everything with Copper.Now all the GrayStuff that is left is the main supply lines. My take on this is; replace with Copper and FORGET all about leaks. None of the copper has had any problems whatsoever,and I did all the work myself with NO help from anyone. The key to my rant is what bcbouy said "easy to repair"- there are NO repairs with Copper,when done correctly.