One problem related to brake antilock wheel speed sensor has been reported for the 2004 GMC Sierra 2500. The most recently reported issues are listed below. Please also check out the statistics and reliability analysis of the 2004 GMC Sierra 2500 based on all problems reported for the 2004 Sierra 2500.
When slowing using the brake system, the vehicle actually loses braking effectiveness for a short time (less than 1 second) just before the vehicle actually stops- at about 3 to 5 mph. The is then a pulsing feel in the brake pedal and the vehicle then stops. The point is that there is actually a momentary loss of braking effectiveness in the last few moments of a normal stop. Fortunately, them, only "low speeds" are involved; unfortunately, this is very unsettling when stopping at a busy intersection or in congested traffic. It leaves one wondering if/when the brake function will actually fail altogether. I had the problem investigated by my dealer and was told that it involved the anti lock sensors which may be fixed by cleaning. If that wouldn't fix the problem, the sensors would have to be replaced. Costs: cleaning $160; replacement: $1250. Cleaning did not correct the problem.