i did a search on this forum and i think it is the master cylinder is bad but i will discribe the problem to see what yall think.i have a 96 3500.the brake pedal is very soft and pulls to the left and the brake light comes on.when i let off and press the pedal again the truck stops good ,pulls hard to the left and the brake light goes out.the brake fluid looks dirty and i bled the lines twice.it seems to be getting worse.also there is no loss in fluid.

 

Check for leaks anyway. The light will come on if there's a pressure bias in the proportioning valve block(right below the master cyl with a white wire). From the pull, it sounds like a leak or blockage in a line, which could cause both.

The other thing I have seen a LOT with these trucks for some reason is brake pads causing pull only when braking. The diagnosis does not include the light coming on though. If the light didn't come on, then I would say swap the pads from right front to left and left front to right. The pull will change to the right if the pads are the issue.