My 01 3500 4x4 Diesel brake pedal goes rock hard at times mostly happend when I need it most like quick stop both feet on pedal and truck barely stops. Anyone else had this problem?

I haven't had the same thing, but it sounds like you have a problem with the master cylinder...

I had this same problem with my Ram. Turns out there is a rubber hose (vaccum line maybe?) that runs from your Master Cylinder to your Engine Block. Make sure this hose has no cracks and is securely connected. Mine was unhooked, i plugged it back in and whalaa, perfect brake pressure again....and good stopping power. If this hose is good, then I'd say your master cylinder needs to be replaced.

I know I may be far from home, but i have had similar issues in other vehicles, and I concur that you sould check your vacuum hose on your vacuum booster, and if that doesn't do it, go on to the master cylinder

Yes, the vacuum booster. The hose I was talking about that came unhooked on my truck went from the Intake to the Vacuum Booster, not the master cylinder. I apologize, I was really tired when I posted my first response.

Diesels use a vacuum pump (diesels have no vacuum of their own), you may need to check the pump for proper operation and then check all of the vacuum lines going to the accessories.