I have a '99 1500 4x4. The abs and brake lights both came on. brakes sink but build up after 2-3 pumps. fluid is good, I checked under truck and behind each wheel and there no leaks or in the lines. the fluid has not been low. less than 1 year ago, I changed the rear ABS sensor on the rear end cuz the speedometer wasn't registering until it hit 30 mph so it shouldn't be that. I read to brake while in reverse to reset the back brakes; that did not work and to press the brakes while the engine is off as that may indicate a master cylinder going out; the brakes started going down but built up normal pressure after 2-3 pumps. Just like when the truck is running. Any input is appreciated as I respect the collective knowledge on this site.