Have same issue. No ABS lite on , on my 2003 chevy Blazer. Most of the time when I brake the ABS comes on and there is never a ABS Lite. So which sensor could be bad? Also not that it is the ABS because I hear the pump come on.
There are 4 things that can cause this:
Faulty ABS sensor
Fault reluctor ring
Faulty ABS pump
You can do a visual inspection of the sensor, reluctor ring and wiring.
I've seen it quite often where the wiring has rubbed through and every now and again it touches the wheel or other metal object, the ABS ecu then sees this as the wheel being stopped so activates the ABS.
It is also quite common for corrosion to build up behind the ring, the ABS ecu also sees this as the wheel being stopped.
Sometimes but rarely it can be a fault with the pump/ecu.
Without any diagnostic equipment all you can do is inspect each corner one by one.
Ideally you need a diagnostic scanner and view live data, you will see one of the sensor speeds will drop out every now and again.
Low speed ABS triggering is likely caused by a rusty sensor ring.
As the wheel slows the signal from a rust damaged ABS ring becomes attenuated such that the ECU can no longer see the wheel spinning and thinks the wheel has locked.
With increased wheel speed the ferrous ABS ring is able to generate a significantly measurable signal as it passes across the magnetic pickup for the ABS. So I would recommend replacing if it looks in any way out of shape.