Rbake line block. Axle relief valve. What size?

Hi All.
I am a newbee.
I used to wrench on VW beetles about 20 years ago, but have not wrenched since.

I have a 200 Wrangler Sahara (Stock) I purchased new.
It has 84,000 miles on it.
Blew a (steel) brake line.
Looked underneath and all the brake lines are really bad.
So I am attempting to replace them all!

I believe that I have all of the parts.
I am starting with the rears.
Everything is so rusted that I cannot tell what the actual size of all the
bolts and fittings are. I am aware that I can simply cut the old ones off
(which I will do) but I cannot tell what size the bolt head is that is holding
the rear brake line 'block' and axle relief valve to the rear axle ( I believe
this is the actual relief valve?)
(pictured here:
http://ift.tt/1Dx8pSv)

It is so rusty (and there is very little room to get to it) I cannot tell what
size it is. I am afraid of stripping it.

So, does anyone know the correct size of the of the relief valve head that is
holding the rear brake line block to the axle? Is it SAE or metric?

Any help is much appreciated.

Rbake line block. Axle relief valve. What size?