Flare vs Double Flare?

[jorgensensc]

So I'm using a later model booster setup for power brakes and it puts the prop valve at the front crossmember. I need to shorten the rear brake line to make it fit. I borrowed a flaring kit from advanced in order to shorten the line. I thought I would practice on a pice of brakeline first. Good thing I did. I can only get the tool to do a regular flare. When i try and use the adapter to do a double flair first, it just pushes the brake line out of the flaring bar. Do I need to do a double flare, or will a single flare work okay? I just need to shorten the one line, so I really don't want to buy a whole flaring kit. Any suggestions?
Shawn