When braking the ABS system activates just.

False activation is real common on GM products. In the case of your vehicle rust has built up between the wheel speed sensors and wheel bearings. Those sensors don't put out much of a signal to begin with, and when rust pushes the sensors further away, the signal goes even lower. That causes them to drop out at low speeds making the ABS Computer think that wheel is locking up. Have the abs system and brakes checked, abs system will need to be scanned for codes.