What would you do if you test drove a car from a dealership with a blown brake line?

When it comes to cars I know my stuff I'm a ase certified tech and I've never bought a car from a dealer I have always bought from private party. There is a truck that is for sale in their lot and I looked at it and it had its typical problems. I asked if it was inspected he said yes so I assumed it was road worthy. I got in it and the pedal went to the floor slow. So I was thinking a bad master cylinder. The abs liggt was on and so was the break warning light. I pulled out the lot test drove it and I seen the lights were on and I pulled up on the emergency brake release to see if that was making the light come on. It wasn't. So I went back. I noted all the work I seen it needed and I low balled the guy he was asking 5 grand I offered 1500(I'm saving my back) so he says no and only goes down 300 bucks so I left him my name and number so as I was leaving I seen a puddle underneath of it and was wondering what it was.later on that night I went back to see what it was and come to find out the break line was blown. Do I go back and use it to my advantage to get the vehicle at a low price or do I not use it. I can fix all the problems but I guess what I am asking is can I black mail/threaten to sue for putting my life In danger for telling me the vehicle was inspected and leting me drive it with a blown break line?