water line connections, some have gaskets, some not?

[fishkayaks]

Hi everyone!

While replacing my water pump in my 2013 36FB I jostled a hot water line just enought to cause it to develop it's own slow leak. It was at a connection where a 1/2 inch high pressure hot water line hand tightens onto a metal back flow valve.

That particular connection uses a cone shaped washer which I replaced first, and all's well.

I've noticed some of these water line connections do not have such a washer, and others do. In fact, the new female hose connector I bought in case I needed a replacement does not use a gasket.

Are such washers only needed for plastic to metal connections, or just hot water connections, or should they be used on all connections...?