I have a 1989 c2500. I started it up today and was checking how firm the brake pedal was, as I was pumping, I heard a hissing sound and it is now leaking brake fluid. Any thoughts, master cylinder, brake line, anything?

 

This is just MY opinion, but I would imagine you had a bad brake line and when you started pumping the brakes the extra pressure on the lines caused one of them to crack. I would get someone to pump the brakes while I checked out all the lines, the last thing you wanna do is be driving 70 and try to slow down to realize you no longer have brakes.

 

yep.
you just gatta get down and find where it is coming from. good luck.