My brake light on my dash just came on and then all of a sudden my brake pedal went real soft and my truck would hardly stop. Is this the master cylinder? If so can someone tell me where I can find a diagram to find and fix it? If it might be something else please let me know!! Thanks

 

I'm not a brake expert, but it's also possible that your problem is with your vacuum pump. Since diesels don't produce vacuum like gassers, there's a belt driven mechanism on the front-left corner of the engine that gives you the vacuum needed to operate the brakes, etc. Mine is getting ready to go out on me, but I've been putting off replacement until it stops raining in Seattle.
My symptoms: primarily when the engine isn't fully warmed up, if I step on the brakes firmly, the pedal will feel soft, there will be some hissing sounds coming from the engine compartment, and the brake light will come on. As soon as I step on the throttle, the light will go back out. For what it's worth, it seems to happen more frequently on cold mornings.
Hopefully, someone else with a little more brake experience will come along and give their diagnosis.
Good luck.