Greetings,
Vehicle: As stated in the title.

To make it shorter, lost brakes on a snowy subfreezing night. Limped home at slow pace relying entirely on the hand brakes.
Murphy's Law never fails. First 3 wheels were fine. After removing the 4th wheel, rear-left, I found the leak.

It's from a short link (brake line) that connects between the rear brake cylinder (RL) and the main line, rusted out.
It looks somewhat like a question mark, or like the hook of a clothes hanger.
I thought it's going to be a simple job to replace it.
So far the 10 mm nut that connects this short pipe to the cylinder has refused to budge.
I have used a quality wrench and then locking (vise grip) pliers to no avail. I tried the rocking back & forth too.
I had sprayed penetrating oil multiple times, but nothing seems to make a difference.

Now I'm thinking of sawing the pipe off right at the nut and use an air impact wrench, but I thought I ask for some ideas first
.
I am assuming this nut loosens counter-clockwise, which is what I have been trying.
It's # 14 in the figure below (lower-right area).
Thank you in advance.

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The leak is exactly at the end of the pipe where the nut is (in the red circle)

 

are you using a flare wrench or just a plain open ended wrench ?

 

I'm using a regular Snap-On wrench. I doubt very much that using a different wrench would have made a difference, given that the nose-grip, locking pliers couldn't do anything..

 

Try with a flare wrench and see what happens

just pick up a set of metric flare wrenches from Harbor Freight

you can also use a wire brush and try to scrub enough of the rust away so it's not seized or even a wire brush attached to a drill or dremel

 

Thanks. The set should be on its way now.