Leaky pipe under house

I am trying to figure out what is leaking under my house. I can tell what it is and where it's coming from, but I don't know what it's called or what size to purchase (it does have a number on the side of it). Is there a way to find out or do I just need to take it apart? Most plumbing I've ever done is replaced the guts in my toilet, so any info would be awesome... as I have no idea what to do. Rather learn than call a plumber at this point...

Thanks!

Can you take a pic and post it here? If not, I have no idea what you would be talking about.

yes, a picture,

I believe it is the pressure valve.

It looks as though either the union on the valve or the male adaptor that is threaded into the union is leaking.